U.S. patent number 5,247,382 [Application Number 07/753,634] was granted by the patent office on 1993-09-21 for polarization switching light source, optical receiver, and coherent optical transmission system.
This patent grant is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Nobuo Suzuki.
United States Patent |
5,247,382 |
Suzuki |
September 21, 1993 |
Polarization switching light source, optical receiver, and coherent
optical transmission system
Abstract
A polarization switching light source includes a semiconductor
laser for outputting laser output light, a phase modulation circuit
for subjecting phase modulation of a predetermined cyclic pattern
to the semiconductor laser by supplying a pulse current thereto, a
beam splitter for dividing output light from the semiconductor
laser into first and second branch output light components having
substantially the same power, an optical delay member for delaying
the first branch output light with respect to the second branch
output light by a predetermined amount, and a mixing member for
mixing the first branch output light component, delayed by the
optical delay member, with the second branch output light component
while their polarization states are caused to be orthogonal. A
delay time of the first branch output light component with respect
to the second branch output light component is set to be a fraction
of an integer of the period of the phase modulation.
Inventors: |
Suzuki; Nobuo (Tokyo,
JP) |
Assignee: |
Kabushiki Kaisha Toshiba
(Kawasaki, JP)
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Family
ID: |
16846599 |
Appl.
No.: |
07/753,634 |
Filed: |
August 30, 1991 |
Foreign Application Priority Data
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Aug 30, 1990 [JP] |
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2-226531 |
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Current U.S.
Class: |
398/201; 398/161;
398/188; 398/205 |
Current CPC
Class: |
H04B
10/532 (20130101) |
Current International
Class: |
H04B
10/135 (20060101); H04B 010/00 (); H04B
010/06 () |
Field of
Search: |
;359/192,191,189,194,183,156,132,133,140,164,139,137 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0329186 |
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Aug 1989 |
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EP |
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0357799 |
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Mar 1990 |
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EP |
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0052530 |
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Mar 1988 |
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JP |
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2213026 |
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Aug 1989 |
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GB |
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Other References
Electronics Letters, vol. 24, No. 15, Jul. 21, 1988, pp. 974-976,
I. M. I. Habbab, et al., "Phase-Insensitive Zero-If Coherent
Optical System Using Phase Switching"..
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Primary Examiner: Coles, Sr.; Edward L.
Assistant Examiner: Negash; Kinfe-Michael
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt
Claims
What is claimed is:
1. An optical circuit which includes a polarization switching light
source, said optical circuit comprising:
a semiconductor laser for outputting laser output light;
phase modulation means for supplying a pulse current to said
semiconductor laser to subject a phase modulation of a
predetermined cyclic pattern to said semiconductor laser;
branch means for dividing the output light from said semiconductor
laser into first and second branch output light components having
substantially the same power;
delay means for delaying the first branch output light component
with respect to the second branch output light component, by a
predetermined amount; and
means for mixing the first branch output light component delayed by
said delay means with the second branch output light component
while polarization states thereof are caused to be orthogonal,
wherein a delay time of the first branch output light component
with respect to the second branch output light component is set to
be a fraction of an integer of a period of the phase
modulation.
2. An optical receiver comprising:
a local oscillator, including a semiconductor laser for outputting
laser output light, for generating a local oscillation output light
having at least two polarization states corresponding respectively
to at least two first time slots obtained by dividing a second time
slot corresponding to a one-bit signal;
optical receiving means for receiving, as a reception signal, a
beat component resulting from interference between signal light
from a transmission light source and the local oscillation output
light of said local oscillator which corresponds to each of the
first time slots;
detection means for detecting said reception signal in each of a
plurality of first time slots, and outputting at least two
detection outputs corresponding to said first time slots; and
means for adding the detection outputs in the first time slots
within the second time slot;
phase modulation means for supplying a pulse current to said
semiconductor laser to subject a phase modulation of a
predetermined cyclic pattern to said semiconductor laser;
branch means for dividing the output light from said semiconductor
laser into first and second branch output light components having
substantially the same power;
delay means for delaying the first branch output light component
with respect to the second output light component, by a
predetermined amount; and
means for mixing the first branch output light component delayed by
said delay means with the second branch output light component
while the polarization states thereof are caused to be
orthogonal,
wherein a delay time of the first branch output light component
with respect to the second branch output light component is set to
be a fraction of an integer of a period of the phase
modulation.
3. The optical receiver according to claim 2, wherein said optical
receiving means includes means for inhibiting signal detection at
the instant when a frequency variation accompanying a phase
variation of said local oscillation light appears in an electrical
beat signal.
4. The optical receiver according to claim 2, which includes AFC
means for detecting a difference between a beat frequency and a
preset value thereof and controlling a oscillation frequency of
said polarization switching light source to set the beat frequency
to be a predetermined value only when a frequency variation
accompanying a phase variation of the local oscillation light does
not appear in a beat signal.
5. An optical receiver comprising:
a local oscillator having means for performing phase modulation of
a semiconductor laser by supplying a pulse current thereto, said
local oscillator performing fixed phase switching between .pi./2
and -.pi./2 upon switching of phase sub-time slots obtained by
dividing a time slot corresponding to a one-bit signal into not
less than three portions;
optical receiving means for receiving, as a reception signal, a
beat component resulting from interference between output light
from said local oscillator and signal light from an optical
transmitter;
delay means for dividing the reception signal received by said
optical receiving means into two output components, and delaying
one output component by a predetermined amount;
a 90.degree. hybrid circuit for shifting a phase of one of the two
output components, one of which is delayed by said delay means, by
90.degree., and adding the phase-shifted output component to the
other output component; and
a detector for detecting an output from said 90.degree. hybrid
circuit in each sub-time slot.
6. The optical receiver according to claim 5, wherein said optical
receiving means includes means for inhibiting signal detection at
the instant when a frequency variation accompanying a phase
variation of the output light of said local oscillator appears in
an electrical beat signal.
7. The optical receiver according to claim 5, which includes AFC
means for detecting a difference between a beat frequency and a
preset value thereof and controlling an oscillation frequency of
said local oscillator to set the beat frequency to be a
predetermined value only when a frequency variation accompanying a
phase variation of the output light of said local oscillator does
not appear in a beat signal.
8. An optical receiver comprising:
a local oscillator having a semiconductor laser and phase
modulation means for performing phase modulation of said
semiconductor laser by supplying a pulse current thereto, said
local oscillator performing phase switching between .pi./2 and
-.pi./2 upon switching of phase sub-time slots obtained by dividing
a time slot corresponding to a one-bit signal into not less than
two portions;
optical receiving means for receiving, as a reception signal, a
beat component resulting from interference between output light
from said local oscillator and signal light from an optical
transmitter;
delay means for dividing the reception signal received by said
optical receiving means into two output components, and delaying
one output component by a predetermined amount;
a 90.degree. hybrid circuit for shifting a phase of one of the two
output components, one of which is delayed by said delay means, by
90.degree., and adding the phase-shifted output component to the
other output component;
a signal path switching circuit coupled before or after said
90.degree. hybrid circuit with respect to the direction of signal
transmission from said transmitter to said receiver; and
a detector for detecting a signal transmitted through said
90.degree. hybrid circuit and said signal path switching
circuit,
wherein image rejection signal reception is performed by switching
said signal path switching circuit in synchronism with each phase
sub-time slot.
9. The optical receiver according to claim 8, wherein said optical
receiving means includes means for inhibiting signal detection at
the instant when a frequency variation accompanying a phase
variation of the output light of said local oscillator appears in
an electrical beat signal.
10. The optical receiver according to claim 8, which includes AFC
means for detecting a difference between a beat frequency and a
preset value thereof and controlling an oscillation frequency of
said local oscillator to set the beat frequency to be a
predetermined value only when a frequency variation accompanying a
phase variation of the output light of said local oscillator does
not appear in a beat signal.
11. A coherent optical transmission system comprising:
at least one optical transmitter having phase modulation means for
performing phase modulation of a semiconductor laser by supplying
alternately positive and negative pulse currents thereto, said
optical transmitter performing phase switching between .pi./2 and
-.pi./2 upon switching of phase sub-time slots obtained by dividing
a time slot corresponding to a one-bit signal into not less than
two portions; and
at least one optical receiver having a polarization switching light
source, including a semiconductor laser, wherein a polarization
switching timing of said polarization switching light source is set
such that the different polarization states appear in the same
phase state in one time slot of signal light, said polarization
switching light source serving as a local oscillator, optical
receiving means for receiving, as a reception signal, a beam
component resulting from interference between output light from
said local oscillator and signal light from said optical
transmitter, a detector for detecting said reception signal of each
sub-time slot, and means for adding detection outputs in new
phase/polarization sub-time slots, which are divided at timings of
polarization switching and phase switching and have different
polarization and phase states, within a time slot,
phase modulation means for supplying alternately positive and
negative pulse currents to said semiconductor laser to perform
phase modulation of said semiconductor laser,
branch means for dividing the output light from said semiconductor
laser into two branch output light components having substantially
the same power;
delay means for delaying one branch output light component with
respect to the other branch output light component by a
predetermined amount, and
means for mixing one delayed branch output light component with the
other branch output light component while polarization states
thereof are caused to be orthogonal,
wherein a delay time of one branch output light component with
respect to the other branch output light component is set to be a
fraction of an integer of a period of the phase modulation.
12. The coherent optical transmission system according to claim 11,
which includes means for detecting a clock by detecting a
pulse-like frequency variation accompanying phase switching of said
optical transmitter.
13. The coherent optical transmission system according to claim 11,
which includes means for changing a pulse waveform, a pulse
pattern, or a polarity by means of pulse-like frequency modulation
for phase switching performed by said optical transmitter and
pulse-like frequency modulation for polarization switching
performed by said local oscillator of said optical receiver.
14. The coherent optical transmission system according to claim 11,
which includes means for identifying and detecting a pulse-like
frequency variation accompanying polarization switching and a
pulse-like frequency variation accompanying phase switching, and
means for controlling the timing of polarization switching by using
the detected signals.
15. The coherent optical transmission system according to claim 11,
wherein said optical receiver includes means for inhibiting signal
detection at the instant when a frequency variation accompanying a
phase variation of one of the signal light and the output light of
said local oscillator appears in an electrical beat signal.
16. The coherent optical transmission system according to claim 11,
which includes AFC means for detecting a difference between a beat
frequency and a preset value thereof and controlling the
oscillation frequency of said local oscillator to set the beat
frequency to be a predetermined value only when a frequency
variation accompanying a phase variation of the output light of
said local oscillator does not appear in an electrical beat
signal.
17. The coherent optical transmission system according to claim 11,
which includes means for controlling said optical transmitters so
as to match all the timings of phase switching of the respective
optical transmitters with each other in the corresponding optical
receivers.
18. A coherent light transmission system comprising:
at least one optical transmitter having phase modulation means for
performing phase modulation of a semiconductor laser by supplying
alternately positive and negative pulse currents thereto, said
optical transmitter performing fixed phase switching between .pi./2
and -.pi./2 upon switching of phase sub-time slots obtained by
dividing a time slot corresponding to a one-bit signal into not
less than three portions; and
at least one optical light receiver for performing image removing
signal reception, said optical receiver having an optical receiving
unit for receiving, as a reception signal, a beat component
resulting from interference between local oscillation light from a
local oscillator and an optical signal from said optical
transmitter, delay means for dividing said reception signal
received by said light receiving unit into two output components,
and delaying one output component by a predetermined amount, a
90.degree. hybrid circuit for shifting a phase of one of the two
output components, one of which is delayed by said delay means, by
90.degree., and adding the phase-shifted output component to the
other output component, and a detector for detecting an output from
said 90.degree. hybrid circuit in each sub-time slot.
19. The coherent optical transmission system according to claim 18,
wherein said local oscillator is constituted by a polarization
switching light source, and includes means for adding detection
outputs having different polarization states in one time slot.
20. The coherent optical transmission system according to claim 18,
which includes means for detecting a clock by detecting a
pulse-like frequency variation accompanying phase switching of said
optical transmitter.
21. The coherent optical transmission system according to claim 19,
which includes means for changing a pulse waveform, a pulse
pattern, or a polarity by means of pulse-like frequency modulation
for phase switching performed by said optical transmitter and
pulse-like frequency modulation for polarization switching
performed by said local oscillator of said optical receiver.
22. The coherent optical transmission system according to claim 18,
which includes means for identifying and detecting a pulse-like
frequency variation accompanying polarization switching and a
pulse-like frequency variation accompanying phase switching, and
means for controlling the timing of polarization switching by using
the detected signals.
23. The coherent optical transmission system according to claim 18,
wherein said optical receiver includes means for inhibiting signal
detection at the instant when a frequency variation accompanying a
phase variation of one of the signal light and the output light of
said local oscillator appears in an electrical beat signal.
24. The coherent optical transmission system according to claim 18,
which includes AFC means for detecting a difference between a beat
frequency and a preset value thereof and controlling the
oscillation frequency of said local oscillator to set the beat
frequency to be a predetermined value only when a frequency
variation accompanying a phase variation of the output light of
said local oscillator does not appear in an electrical beat
signal.
25. The coherent optical transmission system according to claim 18,
which includes means for controlling said optical transmitters so
as to match all the timings of phase switching of the respective
optical transmitters with each other in the corresponding optical
receivers.
26. A coherent light transmission system comprising:
at least one optical transmitter having phase modulation means for
performing phase modulation of a semiconductor laser by supplying
alternately positive and negative pulse currents thereto, said
optical transmitter performing phase switching between .pi./2 and
-.pi./2 upon switching of phase sub-time slots obtained by dividing
a time slot corresponding to a one-bit signal into not less than
two portions; and
at least one optical receiver having an optical receiving unit for
receiving, as a reception signal, a beat component resulting from
interference between local oscillation light from a local
oscillator and signal light from said optical transmitter, delay
means for dividing said reception signal received by said optical
receiving unit into two output component, and delaying one output
component by a predetermined amount, a 90.degree. hybrid circuit
for shifting a phase of one of the two output components, one of
which is delayed by said delay means, by 90.degree., and adding the
phase-shifted output component to the other output component, a
signal path switching circuit coupled before or after said
90.degree. hybrid circuit with respect to the direction of signal
transmission from said transmitter to said receiver, and a detector
for detecting a signal transmitted through said 90.degree. hybrid
circuit and said signal path switching circuit, said optical
receiver performing image rejection signal reception by switching
said signal path switching circuit in synchronism with each phase
sub-time slot of an optical signal generated upon phase switching
of said optical transmitter.
27. The coherent optical transmission system according to claim 26,
wherein said local oscillator is constituted by a polarization
switching light source, and includes means for adding detection
output having different polarization states in one time slot.
28. The coherent optical transmission system according to claim 26,
which includes means for detecting a clock by detecting a
pulse-like frequency variation accompanying phase switching of said
optical transmitter.
29. The coherent optical transmission system according to claim 27,
which includes means for changing a pulse waveform, a pulse
pattern, or a polarity by means of pulse-like frequency modulation
for phase switching performed by said optical transmitter and
pulse-like frequency modulation for polarization switching
performed by said local oscillator of said optical receiver.
30. The coherent optical transmission system according to claim 26,
which includes means for identifying and detecting a pulse-like
frequency variation accompanying polarization switching and a
pulse-like frequency variation accompanying phase switching, and
means for controlling the timing of polarization switching by using
the detected signals.
31. The coherent optical transmission system according to claim 26,
wherein said optical receiver includes means for inhibiting signal
detection at the instant when a frequency variation accompanying a
phase variation of one of the signal light and the output light of
said local oscillator appears in an electrical beat signal.
32. The coherent optical transmission system according to claim 26,
which includes AFC means for detecting a difference between a beat
frequency and a preset value thereof and controlling the
oscillation frequency of said local oscillator to set the beat
frequency to be a predetermined value only when a frequency
variation accompanying a phase variation of the output light of
said local oscillator does not appear in an electrical beat
signal.
33. The coherent optical transmission system according to claim 26,
which includes means for controlling said optical transmitters so
as to match all the timings of phase switching of the respective
optical transmitters with each other in the corresponding optical
receivers.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to coherent optical communication
techniques and, more particularly, to a polarization switching
light source, an optical receiver, and a coherent optical
transmission system.
2. Description of the Related Art
Recently, rapid progresses have been made in coherent optical
communication techniques in which heterodyne detection or homodyne
detection is performed by utilizing the properties of light as a
wave (e.g., T. Okoshi and K. Kikuchi, "Coherent Optical Fiber
Communications", KTK Scientific Publishers, Tokyo, 1988). In
coherent optical communication, a local oscillation source is
prepared on the reception side to receive a beat signal resulting
from interference between signal light and local oscillation light
and appearing in a photodetector. Since the spectrum purity of
signal light or local oscillation light is higher than that in a
conventional direct detection system, optical frequency division
multiplexing (optical FDM) with high density can be realized. The
realization of an optical FDM, in combination with the recent
remarkable advances in the optical amplifier, enables long-range
nonrepeated (or multiple branch) optical communication of a
capacity much larger than that of a conventional system. Such a
technique, therefore, is expected to be applied to various types of
communication systems, e.g., a broad band ISDN, a high definition
optical CATV, and a metropolitan area network (MAN).
Several problems, however, are posed in the practical application
of coherent optical communication. The first problem is associated
with polarization matching; the second problem, phase noise; the
third problem, the band width of a receiver; and the fourth
problem, image components which interfere with the realization of a
high density optical FDM. These problems will be sequentially
described below.
(a) Problems associated with polarization matching
In general, signal light transmitted through an optical fiber
varies in polarization state depending on the temperature of the
fiber, a stress acting thereon, and other disturbances. Assuming
that local oscillation light is in a constant polarization state, a
variation in polarization state of signal light corresponds to the
intensity variation of a beat signal resulting from interference.
In an extreme case, if the polarization states of signal light and
local oscillation light are orthogonal, the resulting beat output
becomes zero. In order to solve this problem, the following means
are proposed:
(1) the use of a polarization maintaining fiber
(2) mechanical or electrooptical control of polarization states
(3) polarization diversity reception
(4) polarization scrambling or polarization switching
However, with regard to (1) the use of a polarization maintaining
fiber, problems are posed in terms of a connection method, cost,
and the like. In addition, with regard to (2) control of
polarization states in a fiber, problems are posed in terms of a
trade-off between infinite tracking properties and complexity of a
controller, reliability, size, difficulty in a multichannel optical
FDM, difficulty in cold start, and the like.
For the above-described reasons, (3) polarization diversity
reception is widely used (e.g., L. G. Kazovsky, "Phase- and
Polarization-Diversity Coherent Optical Techniques", Journal of
Lightwave Technology, Vol. 7. No. 2 pp. 279-292. February 1989). In
this method, two independent polarization components are separately
received (multi-port reception), and the received components are
electrically synthesized. The method, however, requires a
complicated optical system for a receiver, and also requires two
light receivers and two intermediate frequency (IF) circuits. In
consideration of the application of the method to a subscriber's
system such as an optical CATV system, aside from a trunk
transmission system, problems are posed in terms of adjustment of
an optical system, cost, and size. In addition, in multi-port
reception, the characteristics of the respective reception ports,
such as transmission delay time, coupling efficiency, and gain must
be matched with each other.
In contrast to this, in (4) polarization scrambling (T. G.
Hodgkinson, R. A. Harmon, and D. W. Smith, "Polarization
Insensitive Heterodyne Detection Using Polarization Scrambling",
Electronics Letters, Vol. 23, No. 10, pp. 513-514, May 1987) or
polarization switching (I. M. I. Habbab and L. J. Cimini, Jr.,
"Polarization-Switching Techniques for Coherent Optical
Communications", Journal of Lightwave Technology, Vol. 6, No. 10,
pp. 1537-1548, October 1988), the polarization state of signal
light or local oscillation light is changed in a time slot of one
bit, and an average value is obtained. This method has a receiving
sensitivity lower than that of other methods by 3 dB and cannot be
applied to a system of a high data rate, but the receivers are much
simple. Therefore the method is expected to be applied to a
subscriber's system and the like with a relatively low data rate.
The following polarization scrambling (switching) light sources
have been reported:
(a) a polarization bistable semiconductor laser
(b) a polarization switch using an external modulator (in the
method proposed by T. G. Hodgkinson et al.)
(c) a polarization switch using a laser frequency switch (in the
method proposed by I. M. I. Habbab et al.)
The polarization bistable semiconductor laser (a), however, cannot
be suitably applied to coherent optical communication because the
wavelength of output light or the output power generally varies
during a polarization switching operation. The external modulator
(b) is not suitable for practical applications because it has
various drawbacks, e.g., a large insertion loss, a large driving
voltage, difficulty in high-speed switching, poor temperature
characteristics, poor stability, and low reliability. In addition,
the polarization switch based on laser frequency modulation (c)
causes an increase in spectrum line width, resulting in difficulty
in coherent detection. Since the problems with (a) and (b) seem to
be apparent, only the problems with (c) will be described below
with reference to the following examples.
In a polarization switching heterodyne receiver using a frequency
switch of a local oscillation laser, an oscillation frequency
f.sub.L of the local oscillation laser is frequency-modulated by a
rectangular wave to become f.sub.L1 in the first half of a one-bit
time slot and f.sub.L2 in the second half. A local oscillation
laser output passes through a polarization maintaining optical
fiber of birefringence B=n.sub.x -n.sub.y and is mixed with signal
light having a frequency f.sub.S by an optical fiber coupler. A
beat signal (IF signal) based on signal light and local oscillation
light generated by a photodiode is demodulated into a baseband
signal by an IF circuit and a demodulation circuit. In this case,
the main axis (x axis) of the polarization maintaining optical
fiber is inclined at 45.degree. with respect to the polarization
direction of the input local oscillation light. A length L of the
polarization maintaining optical fiber is set to be L=c.sub.0
/(2B.DELTA.f) (where c.sub.0 is the velocity of light in a vacuum
and .DELTA.f=f.sub.L1 -f.sub.L2. If, for example,
B=5.times.10.sup.-4 and .DELTA.f=1 GHz, then L=300 m.
A phase retardation .DELTA..theta.(f) between the x- and y-axis
polarization components of output light from the polarization
maintaining optical fiber is 2.pi.LBf/c.sub.0. The difference in
the phase retardation for two frequencies f.sub.1 and f.sub.2 is
given by
The polarization states (P.sub.1, P.sub.2) of light having a .pi.
difference in .DELTA..theta. are orthogonal. That is, a
polarization switch for output light is realized.
In this method, however, a frequency change occurs together with
polarization switching of output light. When differential phase
shift keying (DPSK) or amplitude shift keying (ASK) is to be
performed, the IF frequency can be fixed to f.sub.IF =(f.sub.1
-f.sub.2)/2 by setting f.sub.S =(f.sub.1 +f.sub.2). In frequency
shift keying (FSK), however, two IF frequencies appear in
accordance with polarization states. In the above case, since two
IF frequencies having a difference .DELTA.f=1 GHz appear, a wide IF
band must be set to cover the two frequencies. For this reason, a
frequency deviation of 2 GH or more must be set. In addition, an
automatic frequency control (AFC) circuit for stabilizing the IF
frequency is inevitably complicated.
The x- and y-axis polarization components of light output from the
polarization maintaining optical fiber has a propagating time
difference represented by .tau.=LB/c.sub.0 =1/(2.DELTA.f). When
polarization switching is to be performed by this method, in order
to sufficiently reduce the time during which light having a
frequency f.sub.1 and light having a frequency f.sub.2 overlap each
other, the transmission time difference .tau. must be set to be
.tau.<<T/2, i.e., the bit rate must be set as R.sub.B
=1/T<<.DELTA./2. In this case, R.sub.B <<1 Gb/s. This
means that a modulation index m must be set to be large in FSK. For
example, at 100 Mb/s, modulation with a frequency deviation of 2
GHz (m=20) is required. The method described above requires a wide
IF band and causes a great deterioration in sensitivity and hence
is not suitable for practical applications.
Furthermore, since the output power of a semiconductor laser
generally varies with frequency modulation, a beat signal is
unbalanced between two polarization states, and the output exhibits
slight polarization dependency. Although a laser structure and a
modulation method may be designed such that frequency modulation
can be performed with a constant output, a driving circuit and an
output feedback circuit become complicated.
(b) Problems associated with phase noise and receiver band
width.
The drawback of the heterodyne detection scheme is that a receiver
having a wide band is required because an IF band is used.
Especially in transmission at a high bit rate, the band width of a
receiver is a bottleneck. In addition, in consideration of the
application of the scheme to a subscriber's system, even at a low
bit rate, requirement of a wide band for a receiver is not
preferable in terms of cost.
In contrast to this, in the homodyne detection scheme which
requires no IF band, no demerits are found in terms of a receiver
band width. However, the scheme is greatly influenced by phase
noise from a light source. In an extreme case, if signal light and
local oscillation light have a phase difference of 90.degree., the
reception output is zero. Although the spectrum line width of a
semiconductor laser is beginning to be decreased, it is still
difficult to realize a full width half maximum of 100 kHz or less
without using, e.g., a special feedback loop or an external
resonator. Such a feedback loop or an external resonator leads to
an increase in the size of a light source, an increase in the
number of portions to be adjusted, a deterioration in stability and
reliability, and an increase in cost. Even if the line width is
reduced by such a means, the phase noise cannot be reduced to zero.
For this reason, an optical phase locked loop (optical PLL) is
required to match the phase of local oscillation light with that of
signal light. Currently, however, such a means is very difficult to
realize.
A great deal of attention, therefore, is paid to a phase diversity
reception scheme (e.g., L. G. Kazovsky, "Phase- and
Polarization-Diversity Coherent Optical Techniques", Journal of
Lightwave Technology, Vol. 7, No. 2, pp. 279-292, February 1989) as
a scheme capable of receiving signal light in the baseband without
using an optical PLL. In this method, components of a plurality of
phases of signal light are independently received, and the received
components are electrically synthesized in the same manner as in
polarization diversity. Similar to a polarization diversity
receiver, however, a complicated arrangement is required, and the
characteristics of the respective ports must be matched with each
other. In the phase diversity reception scheme, the problem of
polarization matching is also posed. The arrangement for
polarization diversity and phase diversity requires at least four
optical receivers.
In order to prevent this problem, phase switching reception may be
employed (I. M. I. Habbab, J. M. Kahn, and L. J. Greenstein,
"Phase-Insensitive Zero-IF Coherent Optical System Using Phase
Switching", Electronics Letters, Vol. 24, No. 15, pp. 974-976, June
1988). Even the phase switching scheme is not free from the problem
of polarization matching. In addition, no method of simultaneously
performing phase switching and polarization switching has yet been
established.
(c) Problems associated with high density optical FDM
The number of channels which can be multiplexed by an optical FDM
is determined by a usable frequency (wavelength) band and an
allowable channel interval. The frequency band is limited by the
tuning range of an local oscillation source or the band of an
optical system. Although the tuning range has been increased with
the advances in variable-wavelength semiconductor lasers, it is not
sufficiently wide yet. A reduction in channel interval is
indispensable for the effective use of a wavelength band.
Especially in heterodyne reception, the presence of an image
component in an IF band limits a channel interval. A reception
scheme (image removing receiver) for removing this image component
has been proposed. However, if this scheme is combined with
polarization diversity, the resulting arrangement is very
complicated. Such an image removing receiver is disclosed in Terumi
Chikama et al., "Optical Heterodyne Image Rejection Receiver", 1989
Spring National Convention Record, the Institute of Electronics,
Information and Communication Engineers of Japan, Part 4, pp.
4-133, 1989. According to this scheme, an optical system has a very
complicated arrangement, and an electrical system also has a
complicated arrangement with four balanced light receivers. The
application of such a scheme to a multichannel optical CATV
distribution system or an optical LAN is very difficult in terms
of, e.g., reliability and cost.
The problems in the practical application of the coherent optical
communication scheme are associated with: (a) polarization
matching, (b) phase noise and a receiver band width, and (c) a high
density FDM. As methods of solving these problems, the methods
based on the following means are regarded to be effective: (A)
polarization diversity, (B) phase diversity, and (C) multi-port
reception using an image removing receiver and the like. However,
in the method based on multiport reception, a complicated
arrangement is required, and the characteristics of the respective
ports must be matched with each other, thus undesirably restricting
allowable characteristic specifications. If two or more of the
methods (A), (B), and (C) are to be simultaneously performed, a
very complicated arrangement is required. The complication of an
optical system causes an increase in the number of portions to be
adjusted, a deterioration in reliability, and a great increase in
cost and hence poses a serious problem in terms of practical
applications. Even in an electronic circuit system, the
complication of the arrangement of a high-frequency circuit poses a
serious problem, as in hetero-dyne reception.
As other methods of solving the problems (a) and (b), (D) a
polarization switching (scrambling) method and (E) a phase
switching method have been proposed. In the conventional switching
methods, however, a high-speed, low-power-consumption operation is
difficult to realize, and the spectrum line width is undesirably
increased. Therefore, many limitations are imposed on the
application of the methods. For example, it is difficult to apply
the methods to FSK. That is the practical value of the methods is
low. In addition, the methods exhibit poor compatibility with image
removing signal reception.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a polarization
switching light source having a simple arrangement and a small
number of limitations.
It is another object of the present invention to provide a light
receiver which enables a new image rejection reception scheme based
on phase switching without complicating an optical system.
It is still another object of the present invention to provide a
coherent light transmission system which has a simple optical
system, is not dependent on polarization, and is resistant to phase
noise.
The gist of the present invention is that the direct phase
modulation technique of a semiconductor laser is used to enable
simple, easy, low-cost coherent switching reception in terms of the
problems of polarization, phase, and image components.
According to the first aspect of the present invention, there is
provided a polarization switching light source comprising (a) a
semiconductor laser, (b) means for performing phase modulation by
supplying a pulse current to the semiconductor laser, (c) a
dividing device for dividing output light from the semiconductor
laser into two branch components having substantially the same
power, (d) a delay means for delaying one branch component with
respect to the other branch component an integer of a phase
modulation period, and (e) a mixing means for mixing the two branch
components, the polarization axes thereof are set to be orthogonal
at the mixing means.
According to the second aspect of the present invention, there is
provided an optical receiver comprising (a) a local oscillation
source, constituted by the polarization switching light source in
the first aspect, the local oscillation source performing
polarization switching upon switching of sub-time slots obtained by
dividing a time slot T corresponding to a one-bit signal into not
less than two portions, (b) a optical receiver for receiving a beat
component, resulting from signal light and local oscillation light,
as an electrical signal, (c) a detection circuit for detecting a
received signal in each sub-time slot, and (d) an adding circuit
for adding detection outputs in the respective sub-time slots
within the time slot T.
According to the third aspect of the present invention, there is
provided an optical receiver comprising (a) a local oscillation
source, having means for performing phase modulation by supplying a
pulse current to a semiconductor laser, the local oscillation
source performing fixed phase switching between .pi./2 and -.pi./2
upon switching of phase sub-time slots TS obtained by dividing a
time slot T corresponding to a one-bit signal into not less than
three portions, (b) an optical receiving unit for receiving a beat
component, resulting from local oscillation light and signal light,
as an electrical signal, (c) means for dividing a beat output from
the optical receiving unit into two output components, and delaying
one output component with respect to the other output component by
a predetermined amount, (d) an electrical 90.degree. hybrid circuit
for the two output components, and (f) a detection circuit for
detecting an output from the 90.degree. hybrid circuit.
According to the fourth aspect of the present invention, there is
provided an optical receiver comprising (a) a local oscillation
source, having means for per forming phase modulation by supplying
a pulse current to a semiconductor laser, the local oscillation
source per forming fixed phase switching between .pi./2 and -.pi./2
upon switching of phase sub-time slots obtained by dividing a time
slot T corresponding to a one-bit signal into not less than two
portions, (b) an optical receiving unit for receiving a beat
component, resulting from local oscillation light and signal light,
as an electrical signal, (c) means for dividing a beat output from
the optical receiving unit into two output components, and delaying
one output component with respect to the other output component by
a predetermined amount, (d) an electrical 90.degree. hybrid circuit
for the two output components, (e) a signal path switching circuit,
arranged on a stage before or after the 90.degree. hybrid circuit,
the signal path switching circuit switching signal paths of the
90.degree. hybrid circuit in synchronism with a phase sub-time
slot, and (f) a detector for detecting a signal passing through the
90.degree. hybrid circuit and the signal path switching
circuit.
According to the fifth aspect of the present invention, there is
provided a coherent optical transmission system comprising at least
one of an optical transmitter and an optical receiver, the first
optical transmitter having means for performing phase modulation by
supplying a pulse current to a semiconductor laser, and the optical
transmitter performing phase switching between .pi./2 and -.pi./2
upon switching of phase sub-time slots obtained by dividing a time
slot T corresponding to a one-bit signal into not less than two
portions, and the optical receiver including a local oscillation
source constituted by a polarization switching light source in
which a timing of polarization switching is set to cause different
polarization states to independently appear in the same phase state
in one time slot of signal light, an optical receiving unit for
receiving a beat component, resulting from light from the local
oscillation source and signal light from the optical transmitter,
as an electrical signal, a detector for detecting a received signal
in each sub-time slot, and means for adding detection outputs in
new phase/polarization sub-time slots, which are divided at timings
of polarization switching and phase switching and have different
polarization and phase states, within a time slot T.
According to the sixth aspect of the present invention, there is
provided a coherent optical transmission system capable of
performing image rejection signal reception comprising at least one
of an optical transmitter and an optical receiver, the optical
transmitter having means for performing phase modulation by
supplying a pulse current to a semiconductor laser, and the optical
transmitter performing phase switching between .pi./2 and -.pi./2
upon switching of phase sub-time slots obtained by dividing a time
slot T corresponding to a one-bit signal into not less than two
components, and the optical receiver including a local oscillation
source, an optical receiving unit for receiving a beat component,
resulting from light from the local oscillation source and signal
light from the optical transmitter, as an electrical signal, means
for dividing a beat output from the optical receiving unit into two
output components, and delaying one output component with respect
to the other output component by a predetermine amount, an
electrical 90.degree. hybrid circuit for the two output components,
and a detector for detecting an output signal from the 90.degree.
hybrid circuit.
According to the seventh aspect of the present invention, there is
provided a coherent optical transmission system capable of
performing image rejection signal reception comprising at least one
of an optical transmitter and an optical receiver, the optical
transmitter having means for performing phase modulation by
supplying a pulse current to a semiconductor laser, and the optical
transmitter performing phase switching between .pi./2 and -.pi./2
upon switching of phase sub-time slots obtained by dividing a time
slot T corresponding to a one-bit signal into not less than two
components, and the second optical receiver including a local
oscillation source, an optical receiving unit for receiving a beat
component, resulting from light from the local oscillation source
and signal light from the optical transmitter, as an electrical
signal, means for dividing a beat output from the optical receiving
unit into two output components, and delaying one output component
with respect to the other output component by a predetermine
amount, an electrical 90.degree. hybrid circuit for the two output
components, a signal path switching circuit, arranged on a stage
before or after the 90.degree. hybrid circuit, the signal path
switching circuit switching signal paths in synchronism with a
phase sub-time slot of the optical transmitter, and a detector for
detecting a signal passing through the 90.degree. hybrid circuit
and the signal path switching circuit.
The optical receiver according to the second, third, or fourth
aspect of the present invention further includes a function for
inhibiting signal detection at the instant when a frequency
variation accompanying a phase variation of local oscillation light
appears in an electrical beat signal.
The optical receiver according to the second, third, or fourth
aspect of the present invention further includes an AFC function
for detecting a difference between a beat frequency and a preset
value thereof and controlling an oscillation frequency of a local
oscillator to set the beat frequency to be a predetermined value
only when a frequency variation accompanying a phase variation of
local oscillation light does not appear in a beat signal.
In the optical receiver in the coherent optical transmission system
according to the sixth or seventh aspect of the present invention,
the local oscillation source has a function, constituted by a
polarization switching light source, for adding detection outputs
having different polarization states in one time slot.
The optical receiver in the coherent optical transmission system
according to the fifth, sixth, or seventh aspect of the present
invention further includes a function for detecting a clock by
detecting a pulse-like frequency variation accompanying phase
switching of the optical transmitter.
In the coherent optical transmission system according to the fifth
aspect of the present invention in which the local oscillation
source is constituted by a polarization switching optical source,
or the coherent optical transmission system according to the sixth
or seventh aspect of the present invention which includes a
function for adding detection outputs having different polarization
states in one time slot, a pulse waveform, a pulse pattern, or a
polarity is changed by pulse like frequency modulation for phase
switching performed by the optical transmitter and pulse-like
frequency modulation for polarization switching performed by the
local oscillator of the optical receiver. In addition, the optical
receiver includes a function for identifying and detecting a
pulse-like frequency variation accompanying polarization switching
and a pulse-like frequency variation accompanying phase switching,
and a function for controlling the timing of polarization switching
by using the detected signals.
The optical receiver in the coherent optical transmission system
according to the fifth, sixth, or seventh aspect of the present
invention further includes a function for inhibiting signal
detection at the instant when a frequency variation accompanying a
phase variation of signal light or local oscillation light appears
in an electrical beat signal.
In this coherent optical transmission system, the optical receiver
further includes an AFC function for detecting a difference between
a beat frequency and a preset value thereof and controlling the
oscillation frequency of the local oscillator to set the beat
frequency to be a predetermined value only when a frequency
variation accompanying a phase variation of local oscillation light
does not appear in an electrical beat signal.
In a transmission scheme for performing optical frequency
multiplexing (optical FDM) transmission by utilizing the coherent
optical transmission system according to the sixth or seventh
aspect of the present invention, control is performed to match all
the timings of phase switching of the respective optical
transmitters with each other in the corresponding optical
receivers.
In the optical receiver according to the first aspect of the
present invention, frequency modulation is performed in a
pulse-like manner with a predetermined pulse pattern having a
period T by directly supplying a pulse current to the semiconductor
laser. In this case, if an angular frequency deviation at time t is
represented by .DELTA..omega.(t), and a pulse is applied in a time
interval between t=t' and t=t'+kg, then phase modulation
represented by the following equation is performed before and after
the application of the pulse: ##EQU1## This semiconductor laser
output light is divided into branch components A and B. The
propagation time of one branch component A is delayed with respect
to the propagation time of the other branch component B by a
predetermined time .tau.. The branch components A and B are then
synthesized in such a manner that their polarization states are
orthogonal. A phase retardation
.DELTA..theta.(t)=.DELTA.A(t)-.DELTA.B(t) between phases
.theta.A(t) and .theta.B(t) of the branch components A and B
changes every time phase modulation based on a pulse current
appears in the branch component A or B. In this case, since a delay
time difference .tau. between the two branch components A and B is
set to be a fraction of an integer of a period T of phase
modulation, a change with a certain pattern periodically occurs at
the period T. If light components to be synthesized have
substantially the same power, since the polarization state of the
synthesized output light changes with .DELTA..theta.(t), a
polarization switching light source can be realized. If a change
amount of one switching operation with .DELTA..theta.(t) is set to
be, e.g., .pi..+-.2n.pi. (n is an integer), two orthogonal
polarization states are switched according to equation (1). If the
system is adjusted such that the orthogonal polarization states
appear at substantially the same probability in the period T, a
constant beat can be obtained from light components having
arbitrary polarization states in terms of an average time
interval.
According to the second aspect of the present invention, there is
provided the coherent optical receiver using the polarization
switching light source according to the first aspect as a local
oscillation source. In this optical receiver, a phase modulation
pattern is set such that independent, different polarization states
of local oscillation light always appear once or more in the
one-bit time slot T of a transmitted signal. Upon this phase
modulation, a polarization state is divided into two sub-time slots
TS or more. If a beat component, resulting from this local
oscillation light and signal light, is received and detected in
each sub-time slot, and the detection outputs in the respective
subtime slots are added (or averaged) within the time slot T, a
signal output having a substantially constant magnitude can be
obtained.
According to the third aspect of the present invention, there is
provided the image rejection coherent optical receiver using the
semiconductor laser capable of direct phase modulation as a local
oscillation source.
The local oscillation source performs phase modulation
corresponding to an absolute value .pi./2 three times or more in
the one-bit time slot T. With this phase modulation, a time slot is
divided into a plurality of sub-time slots, each having a length
T.sub.S. A beat component, resulting from this local oscillation
light and signal light, is received in each sub-time slot. Assume
that the signal light contains a component .omega..sub.S1 having a
frequency higher than a frequency .omega..sub.L of the local
oscillation light and a component .omega..sub.S2 having a frequency
lower than the frequency .omega..sub.L. The phase of the received
beat signal is also shifted by .pi./2 in each sub-time slot. Assume
that the beat output in the first sub-time slot is represented by
##EQU2## where .theta..sub.1 and .theta..sub.2 are the phase
constants based on the difference between the phase of signal light
and that of local oscillation light. The beat outputs in the second
and subsequent sub-time slots are given by ##EQU3## If five or more
sub-time slots are present, the above-described four states are
repeated as I.sub.5 =I.sub.1, I.sub.6 =I.sub.2, . . . In this case,
A.sub.1 and A.sub.2 are constants. The signs of the first and
second terms of the right-hand side of each equation vary depending
on the magnitudes of the frequency of local oscillation light and
that of signal light.
This beat output is divided into two components. One component is
delayed with respect to the other component by T.sub.S. The two
components are then added together while the phase of one component
is shifted by .pi./2. For example, an output X.sub.1 obtained by
adding one branch component, which is delayed by T.sub.S, to the
other branch component whose phase is shifted by 90.degree. is
represented by ##EQU4##
As a result, only beat components represented by .omega..sub.S1 and
.omega..sub.L (.omega..sub.S >.omega..sub.L) appear. Similarly,
outputs from a port X in the subsequent sub-time slots are
represented by ##EQU5##
In contrast to this, if one branch signal of a light receiver
output is delayed by T.sub.S and its phase is shifted by
90.degree., and the resultant signal is added to the other branch
signal, then outputs Y obtained in the respective sub-time slots
are represented as follows: ##EQU6## As a result, only beat
components represented by .omega..sub.S2 and .omega..sub.L
(.omega..sub.L >.omega..sub.S) appear. In order to shift the
phase of a signal by 90.degree. regardless of its frequency, a
phase shift circuit constituted by a differentiating circuit or an
integrating circuit may be used. If such a circuit is to be used,
it is preferable that a beat frequency be equal to or higher than
the reciprocal of the length of a sub-time slot.
In this case, the outputs detected in the respective sub-time slots
are represented as follows: ##EQU7## That is, signal outputs
proportional to power A.sub.1.sup.2 and A.sub.2.sup.2 of the
respective signals can be obtained. In the above equations, P is a
constant. Both output ports X and Y need not be arranged, but at
least one of them is required.
Assume that the length of a sub-time slot is close to the period of
an IF signal. In this case, when an average output in the sub-time
slot is to be obtained, the output may vary depending on the phase
of the IF signal. In order to prevent this variation, f.sub.IF
=.omega..sub.IF /2.pi.=m/T.sub.S ' (m=1, 2, 3 . . . ) or 1/T.sub.S
'<<f.sub.IF is set, where TS' is the length of a sub-time
slot in which outputs are actually detected and averaged.
The optical receiver according to the fourth aspect of the present
invention is a modification of the apparatus according to the third
aspect. In this modification, the local oscillation source performs
phase switching alternately between .pi./2 and -.pi./2. In the
fourth aspect, at least two sub-time slots are required. Under the
same conditions as those in the third aspect, the following light
receiver outputs alternately appear in the respective sub-time
slots: ##EQU8## This beat output is divided into two branch
components. One branch component is delayed with respect to the
other branch component by T.sub.S, and is added to the other branch
component while the phase of one component is shifted by
90.degree.. For example, if a signal obtained by delaying one
branch component by TS is added to a signal obtained by shifting
the phase of the other branch component by 90.degree., outputs X
obtained in the respective time slots are represented as
follows:
Thus, beat components represented by .omega..sub.S1 and
.omega..sub.L (.omega..sub.S >.omega..sub.L) and beat components
represented by .omega..sub.S2 and .omega..sub.L (.omega..sub.L
>.omega..sub.S) alternately appear. Similarly outputs Y obtained
by adding one branch component, delayed by T.sub.S and
phase-shifted by 90.degree., to the other branch component which is
not delayed are:
Therefore, if the output ports X and Y are switched for every
sub-time slot, only a component represented by .omega..sub.S
>.omega..sub.L or .omega..sub.L >.omega..sub.S can be
received. That is, image rejection signal reception can be
performed. Note that switching of the signal paths may be performed
on the input side of the 90.degree. hybrid circuit instead of
performing it on the subsequent stage.
In the second, third, and fourth aspects, when the phase or
polarization of local oscillation light is to be switched, the
light frequency greatly changes. This may cause an error in signal
demodulation or an AFC operation.
In the second, third, and fourth aspects, if the optical receiver
further includes a function for inhibiting signal detection at the
instant when a frequency variation accompanying a phase variation
of local oscillation light appears in an electrical beat signal, an
increase in the number of errors in signal demodulation can be
prevented because a beat signal in a switching state need not be
used for signal demodulation. If the total time required for
switching in the time slot T is represented by .delta.T, since a
light output which can be used for reception is given by
(T-.delta.T)/T, a power penalty is caused. However, the power
penalty can be reduced by decreasing .delta.T/T. Similarly, if the
light receiver further includes an AFC function for detecting the
difference between a beat frequency and a preset value thereof and
controlling the oscillation frequency of a local oscillator to set
the beat frequency to be a predetermined value only when a
frequency variation accompanying a phase variation of local
oscillation light does not appear in a beat signal, a frequency
variation of the local oscillation source can be suppressed because
a beat signal in a switching state need not used for AFC.
According to the fifth aspect of the present invention, there is
provided the coherent light transmission system which can
simultaneously perform phase switching and polarization
switching.
The time slot T corresponding to the reciprocal of a bit rate
R.sub.b is divided into two or more phase sub-time slots, and a
pulse current is supplied to the semiconductor laser of the light
transmitter at the boundary of the phase sub-time slots, thus
performing phase modulation of .delta..theta.=.+-..pi./2. Note that
in the following description, alternate signs are assigned in
.delta..theta.=.+-..pi./2. If the light output electric field
strength of a phase sub-time slot is represented by
.vertline.E.sub.S /.sqroot.2.vertline.cos(.omega..sub.S
t+.theta..sub.S), the light output electric field strength of the
next sub-time slot is given by .+-..vertline.E.sub.S
/.sqroot.2.vertline.sin(.omega..sub.S t+.theta..sub.S). If three or
more phase sub-time slots are present, the above-mentioned two
states alternately appear. Therefore orthogonal phase components
alternately appear in output light from the transmitter.
Output light having a frequency .omega..sub.L from the local
oscillation source of the light receiver alternately assumes two
orthogonal polarization states upon polarization switching. This
defines a polarization sub-time slot. Polarization sub-time slots
are set such that two independent polarization states A and B
always appear in the respective sub-time slots, in one time slot,
which have two independent phase states. Output light with the
polarization state A having an electric field strength
.vertline.E.sub.L /.sqroot.2.vertline.cos(.omega..sub.L
t+.theta..sub.LA) appears in a polarization sub-time slot A,
whereas output light with the polarization state B having an
electric field strength .vertline.E.sub.L
/.sqroot.2.vertline.cos(.omega..sub.L t+.theta..sub.LB) appears in
a polarization sub-time slot B.
This local oscillation light is mixed with light from the optical
transmitter, and the mixed light is received by the optical
receiver. If components, of signal light power, which have the
polarization states A and B are respectively represented by
.xi..sub.A.sup.2 and .xi..sub.B.sup.2, .xi..sub.A.sup.2
+.xi..sub.B.sup.2 =1. In this case, .xi..sub.A and .xi..sub.B
satisfy 0.ltoreq..xi..sub.A .ltoreq.1 and 0.ltoreq..xi..sub.B
.ltoreq.1, respectively. At this time, the following beat output
components appear in correspondence with phase sub-time slots 1 and
2 and polarization sub-time slots A and B: ##EQU9## where P is a
constant representing a loss and a conversion efficiency, and
.omega..sub.IF =.omega..sub.S -.omega..sub.L is an intermediate
frequency and may be set to be 0. If this beat output is
square-law-detected, the following values are obtained: ##EQU10##
where .DELTA..theta..sub.A =.theta..sub.S -.theta..sub.LA and
.DELTA..theta..sub.B =.theta..sub.S -.theta..sub.LB. The sum of
these detection outputs is ##EQU11## Therefore, even at
.omega..sub.IF .about.0, signal reception can be performed
regardless or the phase difference between signal light and local
oscillation light, and this signal reception does not depend on the
polarization state of signal light in the optical receiver.
In practice, although the phase of the light source varies
depending on the phase noise over time, no significant penalty is
caused as long as a phase variation in the time slot T is
sufficiently small. In general, a polarization state gradually
varies as compared with the time slot T. If the frequency f.sub.IF
is sufficiently low, no problems are posed. If, however, the length
T.sub.S of a sub-time slot determined by a phase and polarization
is close to the period of the IF frequency, an output change occurs
in the sub-time slot. If one representative value is obtained from
the average output, the output varies depending on the phase of the
IF frequency. In order to prevent this variation, f.sub.IF
=m/T.sub.S ' (m=1, 2, 3 . . .) may be set where T.sub.S ' is the
length of a sub-time slot in which outputs are actually detected
and an average value is obtained.
According to the sixth aspect of the present invention, there is
provided the coherent optical transmission system which performs
phase switching with image rejection signal reception according to
the third aspect by using the optical transmitter in place of the
local oscillation source. The operation principle of this system is
the same as that of the system according to the third aspect except
that phase switching is performed by the optical transmitter
instead of the local oscillation source. Note that an image
component must also be phase-switched in synchronism with a desired
signal component in the same manner.
According to the seventh aspect of the present invention, there is
provided the coherent optical transmission system which performs
phase switching with image rejection signal reception according to
the fourth aspect by using the optical transmitter in place of the
local oscillation source. The operation principle of this system is
the same as that of the system according to the fourth aspect
except that phase switching is performed by the optical transmitter
instead of the local oscillation source. Note that an image
component must also be phase-switched in synchronism with a desired
signal component in the same manner.
The local oscillation source is constituted by a polarization
switching light source and has a function for adding detection
outputs with different polarization states in one time slot. The
light receiver of the coherent optical transmission system
according to the sixth or seventh aspect is used for the coherent
optical transmission system constituted by a combination of the
image rejection function and the polarization switching function
according to the second aspect. In this case, if phase switching
for removing image components is performed by the optical
transmitter while polarization switching is performed by the local
oscillation source, image rejection signal reception can be
performed without depending on the polarization state of signal
light reaching the optical receiver.
According to the fifth, sixth, or seventh aspect of the present
invention, there is provided the coherent optical transmission
system wherein the optical receiver further includes a function for
detecting a clock by detecting a pulse-like frequency variation
accompanying phase switching of the optical transmitter, thus
extracting a clock from a frequency variation of an optical signal.
When a clock is to be extracted from a signal, since no periodic
component appears in a clock if the value of the signal does not
change, it is difficult to perform stable clock extraction. In the
systems according to the fifth, sixth, and seventh aspects, signal
light is frequency-modulated in the form of a pulse to perform
periodic phase switching regardless of the value of the signal. By
using such signal light, a stable clock extracting operation can be
performed regardless of the value of a signal.
The coherent optical transmission system according to the fifth
aspect of the present invention in which the local oscillation
source is constituted by a polarization switching light source, or
the coherent optical transmission system according to the sixth or
seventh aspect, which has a function for adding detection outputs
with different polarization states in on time slot, includes a
function for changing a pulse waveform, a pulse pattern, or a
polarity by means of pulse-like frequency modulation for phase
switching performed by the optical transmitter and pulse-like
frequency modulation for polarization switching provided for the
local oscillator of the optical receiver. In addition, the optical
receiver includes a function for identifying and detecting a
pulse-like frequency variation accompanying polarization switching
and a pulse-like frequency variation accompanying phase switching,
and a function for controlling the timing of polarization switching
by using the detected signal. In this case, since the signal light
is frequency-modulated in the form of a pulse for phase switching
while the local oscillation light is frequency-modulated in the
form of a pulse for polarization switching, both pulses appear in
the beat signals. If the waveform, pulse pattern, or polarity of
each frequency-modulated pulse is changed in advance, the two types
of frequency modulation can be discriminated from each other upon
detection of a frequency variation, thus allowing control of the
timing of polarization switching, extraction of a clock, and the
like.
In the coherent optical transmission system according to the fifth,
sixth, or seventh aspect of the present invention, when the phase
or polarization of local oscillation light or signal light is to be
switched, an optical frequency greatly varies, resulting in a
signal demodulation error or an AFC operation error. For this
reason, the system further includes a function for inhibiting
signal detection at the instant when a frequency variation
accompanying the phase variation of signal light or local
oscillation light appears in an electrical beat signal. In this
system, a beat signal in a switching state is not used for signal
demodulation so as to prevent an increase in the number of errors
in signal demodulation. If the total period of time required for
switching in the time slot T is represented by .delta.T, since a
light output used for signal reception becomes (T-.delta.T)/T, a
power penalty is caused. However, by sufficiently reducing
.delta.T/T, the power penalty can be reduced. Similarly, in the
system further including the AFC function for detecting the
difference between a beat frequency and its preset value and
controlling the oscillation frequency of the local oscillator to
set the beat frequency to be a predetermined value only when a
frequency variation accompanying the phase variation of local
oscillation light does not appear in an electrical beat signal, a
beat signal in a switching state is not used for AFC of the local
oscillation source to suppress the frequency variation of the local
oscillation source.
If the optical receiver of the coherent optical transmission system
according to the fifth, sixth, or seventh aspect of the present
invention is applied to an optical FDM, since the optical frequency
greatly changes upon switching of the phase of signal light, this
frequency change may interfere with a signal in another channel. In
addition, in the systems according to the sixth and seventh
aspects, if switching of the phase of an image signal adjacent to a
desired signal is not performed in the same manner as that of the
phase of the desired signal, an image removing operation cannot be
performed. This problem can be solved by synchronizing the phase
switching operations of the respective optical transmitters.
Frequency variations simultaneously occur in the respective
channels. Therefore, a signal error in a given channel due to a
frequency variation in another channel can be prevented by
combining a function for synchronizing phase switching and a
function for inhibiting signal detection at the instant when a
frequency variation accompanying the phase variation of signal
light or local oscillation light appears in an electrical beat
signal. In addition, a combination of the function for
synchronizing phase switching and the above-mentioned AFC function
can prevent an AFC error in a given channel due to a frequency
variation in another channel. Furthermore, in the system according
to the sixth or seventh aspect, image rejection signal reception
can be performed through any channels.
Additional objects and advantages of the invention will be set
forth in the description which follows, and in part will be obvious
from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute
a part of the specification, illustrate presently preferred
embodiments of the invention, and together with the general
description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
FIG. 1 is a block diagram showing a schematic arrangement of a
polarization switching light source according to an embodiment of
the present invention;
FIGS. 2A and 2B are views for explaining the structure and
operation of a semiconductor laser used in the embodiment in FIG.
1;
FIGS. 3A to 3L are graphs for explaining the operation principle of
the polarization switching light source in FIG. 1;
FIG. 4 is a block diagram showing a polarization switching light
source as a module;
FIGS. 5A to 5C are graphs for explaining the first modulation
method of the polarization switching light source;
FIGS. 6A to 6D are graphs for explaining the second modulation
method of the polarization switching light source;
FIGS. 7A to 7C are graphs for explaining the third modulation
method of the polarization switching light source;
FIGS. 8A to 8C are graphs for explaining the fourth modulation
method of the polarization switching light source;
FIGS. 9A and 9B are graphs for explaining the fifth modulation
method of the polarization switching light source;
FIGS. 10A and 10B are graphs for explaining the sixth modulation
method of the polarization switching light source;
FIG. 11 is a block diagram showing a schematic arrangement of a
heterodyne optical receiver according to an embodiment of another
aspect of the present invention;
FIGS. 12A and 12B are block diagrams showing a schematic
arrangement of a polarization diversity heterodyne optical receiver
according to an embodiment of still another aspect of the present
invention;
FIGS. 13A and 13B are block diagrams showing a schematic
arrangement of a coherent optical transmission system according to
an embodiment of still another aspect of the present invention;
and
FIG. 14 is a block diagram showing a schematic arrangement of a
coherent optical transmission system according to another
embodiment of still another aspect of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the polarization switching light source, optical
receiver, and coherent optical transmission system of the present
invention will be sequentially described below with reference to
the accompanying drawings.
First Embodiment: Polarization Switching Light Source
As shown in FIG. 1, this polarization switching light source
comprises a semiconductor laser 1, a driver 2 for supplying a bias
current and a pulse current to the semiconductor laser 1, a lens 3
for collimating output light from the semiconductor laser 1, an
optical isolator 4 having an isolation ratio of 60 dB or more, a
1:1 beam splitter 5 for splitting output light from the optical
isolator 4 into two branch components, an optical system 6 for
adjusting the lengths of the two optical paths, split by the beam
splitter 5, to have a predetermined optical path difference
.delta.1, a .lambda./2 plate 7 for rotating the polarization of one
branch component through 90.degree., a polarization beam splitter 8
for synthesizing the two branch components, an optical system and a
laser stabilizing system accompanying these components, and the
like. The components from the beam splitter 5 to the polarization
beam splitter 8 constitute an optical system 9 similar to a Mach
Zehnder interferometer. This optical system 9 is characterized in
that one branch light component is delayed, and when the two branch
light components are synthesized, their polarization states are
orthogonal to each other.
The semiconductor laser 1 used in this case is a three-section
phase-shifted distributed feedback (DFB) InGaAs/InGaAsP multiple
quantum well (MQW) semiconductor laser having a cavity length of
1.5 mm and formed on an n-InP substrate 10, as shown in FIG. 2A. An
active layer 11 has a multiple quantum well structure having 12
wells, which is formed by alternately stacking InGaAs layers 11a,
each having a width of 7.0 nm, and InGaAsP (a composition for a
wavelength of 1.3 .mu.m) barrier layers 11b, each having a width
8.0 nm. Optical waveguides 12 and 13 are formed on the lower and
upper surfaces of this multiple quantum well active layer 11, while
a first order diffraction grating 14 is formed on one waveguide 13.
The active layer 11 and the optical waveguides 12 and 13 are formed
between a p-InP cladding layer 15 and an n-InP cladding layer 16.
Holes and electrons are respectively injected from upper and lower
electrodes 17 and 18 into the active layer 11 and the optical
waveguides 12 and 13 through the cladding layers 15 and 16. A
.lambda./4-phase shift region 19 is formed in a diffraction grating
portion in the middle of the semiconductor laser 1. On both facets
of the semiconductor laser 1, anti-reflection coatings 20, each
having a reflectivity of 0.3% or less with respect to a laser beam,
are formed. The upper electrode is constituted by a central portion
17a having a length of about 700 .mu.m and two end portions 17b and
17c, each having a length of about 400 .mu.m. High-resistance
regions 21, each having a width of 4 .mu.m, are formed between the
respective electrode portions. Each region 21 has a resistance of 1
M.OMEGA. or more. The semiconductor laser 1 is mounted on a heat
sink and is packaged together with a Peltier element and a thermal
sensor. The temperature of the semiconductor laser 1 is stabilized
with a precision of .+-.0.01.degree. C. by a temperature feedback
circuit.
In this semiconductor laser 1, by adjusting a bias current to be
supplied to each electrode, an FM modulation efficiency exhibiting
a flat curve of about 0.8 GHz/mA in the range of modulation
frequencies from 100 kHz to 15 GHz is obtained, as shown in FIG.
2B. At this time, an FM modulation current is supplied to only the
central electrode 17a of the semiconductor laser 1. The
semiconductor laser 1 oscillates in a single mode having a
wavelength of about 1.55 .mu.m while it is biased in this manner.
The oscillated light is linear polarization light. A side mode
suppression ratio is controlled to be 40 dB or more; and an
oscillation spectrum full-width half maximum, 1 MHz or less, owing
to the effects of suppression of return light by means of the
optical isolator 4, temperature stabilization, bias current
stabilization, and packaging resistant to external conditions, in
addition to the superior characteristics of the semiconductor laser
1.
FIG. 3A shows the waveform of a pulse current to be supplied to the
central portion of the semiconductor laser 1 through the driver 2.
Positive and negative current pulses, each having an amplitude of
3.125 mA and a width .delta..tau.=100 ps, are alternately applied
every .tau./2=500 ps. As a result, as shown in FIG. 3B, a laser
oscillation frequency of 2.5 GHz is modulated into alternate
positive and negative pulses, each having the width .delta..tau..
If this modulated signal is expressed as the phase of a center
oscillation frequency, switching occurs between 0 and .pi./2 every
.tau./2, as shown in FIG. 3C. For the sake of descriptive
convenience, the above-mentioned ideal pulse waveform is assumed in
the following description. The gist of this embodiment, however, is
that the phase of output light is switched between 0 and .pi./2
within a period of time shorter than .tau./2. Therefore, the
present invention is not limited to the above-mentioned modulated
waveforms of the current and oscillation frequency. In practice,
since it is difficult to obtain the modulated waveform of a current
or an oscillation frequency as a perfect rectangular wave, the
driver 2 is adjusted such that the phase of output light is changed
between 0 and .pi./2 within a short period of time every .tau./2,
as shown in FIG. 3C. The light electric field of the output light
at this time can be expressed as ##EQU12## where .omega. is the
oscillation light angular frequency, and .theta.(t) is the phase
shown in FIG. 3C: ##EQU13## where m is an integer. Between
t-m.tau.=.tau./2-.delta..tau..about..delta..tau. and
.tau.-.delta..tau..about..tau., the phase continuously changes
between these values.
The optical path difference k1 of the optical system 9 is set such
that one optical path B is delayed with respect to the other
optical path A by .tau./2. With a delay of .tau./2=500 ps, if the
optical paths are in the air, the optical path difference .delta.1
is 15 cm according to the following equation:
where c.sub.0 is the velocity of light in a vacuum. If a light
electric field E.sub.A generated by the polarization beam splitter
8 in the optical path A is represented by ##EQU14## then, a light
electric field E.sub.B generated by the polarization beam splitter
8 in the optical path B is given by ##EQU15## where
In this case, the phase .phi..sub.A =0 is set for the sake of
descriptive convenience. Assume that the light electric fields
E.sub.A and E.sub.B are respectively polarized in the x and y
directions. A phase difference .delta..phi. between the optical
paths A and B changes from -.pi. to .pi. every time the optical
path difference .delta.1 changes in accordance with a wavelength
period. FIG. 3D is a graph showing .theta..sub.A (t) (solid curve)
and .theta..sub.B (t) (dotted curve) when .delta..phi.=0. FIGS. 3E
and 3F, and 3G and 3H show the states of the electric fields of
polarization beam splitter output light in time slots (1) and (2).
As is apparent from FIGS. 3E to 3H, the output light is switched
between two orthogonal polarization states, i.e., right-circularly
polarized light and left-circularly polarized light every .tau./2.
However, an intermediate polarization state appears between
0<t-m.tau.<.tau./2-.delta..tau. and
.tau./2<t-m.tau.<.tau.-.delta..tau. (m is an integer). FIG.
3I shows this state expressed on the Poincare sphere, which is
based on the expression in S. C. Rashleigh, Journal of Lightwave
Technology Vol. LT-1, No. 2, pp. 312-331, June 1983. Since the
light electric fields E.sub.A and E.sub.B have the same amplitude,
the polarization state of the synthesized output light is present
on a PLQR circular cross section. If
then, .DELTA..theta. corresponds to a polarization angle measured
from a point P toward a point L. Two polarization states located at
positions symmetrical about an origin O on this circumference are
orthogonal to each other. In this case, the two polarization states
correspond to right-circularly polarized light R
(.DELTA..theta.=-.pi./2) in time slot (1) and left-circularly
polarized light L (.DELTA..theta.=.pi./2). In switching, the
polarization states reciprocate on an RPL semicircle. FIG. 3J shows
.theta..sub.A (t) (solid curve) and .theta..sub.B (t) (dotted
curve) when .delta..phi.=-.pi./2. FIG. 3K is a graph showing the
polarization state of output light, in which the polarization state
is switched between two orthogonal linear polarization states P and
Q respectively inclined as 45.degree. and -45.degree. with respect
to the x axis, as P.rarw..fwdarw.L.rarw..fwdarw.Q, every .tau./2.
Similarly, when .delta..phi.=.pi., two circular polarization states
alternately appear every .tau./2, whereas when .delta..phi.=.pi./2,
two orthogonal linear polarization states alternately appear every
.tau./2. Otherwise, the output light is elliptically polarized. In
this case, since the difference between
.DELTA..theta.=.theta..sub.A -.theta..sub.B for
0<t-m.tau.<.tau./2-.delta. .tau. and .DELTA..theta. for
.tau./2<t-m.tau.<.tau.-.delta..tau. is .pi., two orthogonal
elliptical polarization states appear every .tau./2. That is, as
shown in FIG. 3L, polarization states alternately appear at two
points S and T symmetrical about the center on a PLQR
circumference. In general, therefore, when light which is
phase-modulated as shown in FIG. 3C is transmitted through the
optical system 9, the output light alternately exhibits two
orthogonal polarization states every .tau./2, thereby realizing a
polarization switching light source for switching two orthogonal
polarization states at a period .tau..
In the modulation method of the polarization switching light source
of this embodiment, it is important to set the time during which
one of two orthogonal polarization states appears to be equal to
the time during which the other polarization state appears. The
conditions for this setting will be clarified below. The
probability that .DELTA..theta.(t)=.theta.A(t)-.theta.B(t) becomes
.DELTA..theta..sub.0 within t=0.about..tau. is defined as a phase
difference probability density function Pr(.DELTA..theta..sub.0).
In this case, assume that the variable .DELTA..theta..sub.0 of
Pr(.DELTA..theta..sub.0) takes a value satisfying
-.pi.<.DELTA..theta..sub.0 .ltoreq..pi. as a principal value. In
addition, assume that ##EQU16## If .DELTA..theta..sub.1 is an
arbitrary phase difference, an output polarization state for
.DELTA..theta.=.DELTA..theta..sub.1 and an output polarization
state for .DELTA..theta.=.DELTA..theta..sub.1 .+-..pi. are
orthogonal to each other. Therefore, if
is 0 for all values of .DELTA..theta., polarization states
appearing between 0 and .tau. equally take two orthogonal
polarization states. That is, the probabilities that polarization
states appear at two points symmetrical about the center on the
PLQR circumference become equal, and a perfect polarization
switching state can be realized. In the above-described modulation
method, however, unless .delta..tau.=0, polarization states at
transient portions where phases are switched do not cancel each
other, as shown in FIGS. 3G, 3I, and 3J. That is, an imperfect
polarization switching light source is formed. However, if
.delta..tau. is sufficiently shortened to reduce ##EQU17## to a
value sufficiently smaller than 1, the above-described apparatus
can be used as a polarization switching light source in practice.
It is preferable to set R<0.25 in practice. In this embodiment,
R.about.0.2.
Note that upon modulation, the time-averaged laser oscillation
spectrum line width is increased. This increase causes phase
discontinuity of a beat signal, over time, when beating is caused
between different monochromatic light components having similar
frequencies and predetermined polarization states and phases. That
is, in general, the phase of a beat signal as well as its amplitude
vary from time slot (1) to time slot (2). The phase variation of
the beat signal is dependent on the polarization states of
different monochromatic light components. If .delta..tau. is
prolonged, a phase variation per unit time in a transient portion
is reduced, and the average spectrum line width is increased. If
.delta..tau. is shortened, the phase variation becomes steep, the
spectrum line width is reduced but a sideband is produced at a
remote position.
The speed of direct modulation of the semiconductor laser can be
easily increased as compared with a case wherein an external phase
modulator is used. In addition, the polarization switching light
source requires only a small driving current of several mA for a
switching operation, and hence the load of the driver is small as
compared with a high-speed external phase modulator requiring a
driving voltage of several V or more. The switching frequency
(f.sub.sc =1/.tau.) of the polarization switching light source of
this embodiment is 1 GHz. Note that if the period of polarization
switching is sufficiently short as compared with the changing speed
of a target phenomenon, this polarization switching light source
can be regarded as a polarization scrambling light source.
FIG. 4 shows a module equivalent to the polarization switching
light source in FIG. 1. A semiconductor laser 1, a lens 3, and an
optical isolator 4 are housed in a module 41 together with a
Peltier element. The semiconductor laser 1 is basically the same as
that shown in FIG. 2A. Output light from the optical isolator 4 is
coupled with polarization matching to the input fiber of a 1:1
polarization maintaining fiber coupler 45 through a spherical lens
42. Each output of the fiber coupler 45 is connected to one end of
each of two polarization maintaining optical fibers 43 (optical
path A) and 44 (optical path B). The other end of each of the
polarization maintaining optical fibers 43 and 44 is connected to a
fiber-optic polarization beam splitter (working as a coupler) 48
such that the polarization main axes are perpendicular to each
other. The fiberoptic polarization beam splitter 48 is designed to
transfer almost 100% of the light polarized in x axis from one
optical fiber to the other optical fiber and to leave almost 100%
of the light polarized in y axis from the other optical fiber in
the same optical fiber. Therefore, light components propagating
through the two polarization maintaining optical fibers 43 and 44
are output from a single fiber 49. The polarization maintaining
optical fiber 44 is longer than the polarization maintaining
optical fiber 43 by .delta.1=(c.sub.0 /n.sub.eff).times.(.tau./2).
For this reason, in the fiber-optic polarization beam splitter 48,
light propagating through the optical path B 44 is delayed with
respect to light propagating through the optical path A 43 by
.tau./2=500 ps. The value of .delta.1 is 10 cm, provided that an
equivalent refractive index n.sub.eff of an optical fiber is 1.5.
The modulation method, operation, and effect of the semiconductor
laser 1 are the same as those in the previous embodiment.
Note that the range of components to be formed into a module is not
limited to that described above. For example, the range may include
the fiber couplers 45 and 48 and the optical fibers 43, 44, and 49.
The optical fibers may be integrated with the module in the form of
a pigtail. In addition, the fiber-optic polarization beam splitter
need not be used to synthesize two branch light components. For
example, a 10:1 polarization beam splitter or a normal
polarization-independent optical coupler may be used. In this case,
feedback control of polarization switching can be performed by
using the other fiber output light. The semiconductor laser 1 is
not limited to the one shown in FIG. 2A but various types of
semiconductor lasers, e.g., a DBR laser, an external resonator
laser, a double-hetero laser, and a strained quantum well laser,
can be used. As a material for a semiconductor laser, various
combinations of substances, such as GaAs/AlGaAs, GaAs/InGaAlP, and
GaSb/GaAlSb, can be used. Therefore, the apparatus can be applied
to various types of light, e.g., visible light and infrared
light.
Various modulation methods are available as modulation methods of
performing polarization switching with the arrangement of the
polarization switching light source of the present invention.
A modulation method for the polarization switching light source
will be described below with reference to FIGS. 5A to 5C. FIG. 5A
shows the modulation waveform for an oscillation frequency. In this
case, frequency modulation of a frequency deviation of 5 GHz is
performed over a width .delta..tau.=100 ps at intervals of .tau.=1
ns. The phase of local oscillation light is changed by .pi. before
and after the application of the pulse. Since a branch light
component propagating through the optical fiber 44 is delayed by
.tau./2 in the optical system, phases .theta..sub.A (t) and
.theta..sub.B (t) of light electric fields synthesized by the fiber
optic polarization beam splitter (working as a coupler) 48 appear
as shown in FIG. 5B, similar to the above-described case. In this
case, .delta..phi.=0 is assumed. As a result,
.DELTA..theta.(t)=.theta..sub.A (t)-.theta..sub.B (t) is switched
between 0 and c every .tau./2. Since the polarization states of the
light components from the optical paths A and B are perpendicular
to each other in the coupler 48, two orthogonal linearly polarized
light components alternately appear at the output of the coupler 48
every .tau./2. Similarly, in the case of .delta..phi..noteq.0,
.DELTA..theta. changes by .pi. every .pi./2, and two orthogonal
circularly or elliptically polarized light components alternately
appear. Therefore, a polarization switching light source can be
realized regardless of the value of .delta..phi.. However, as FIG.
5C shows changes in polarization state on a PLQR circular cross
section of the Poincare sphere, although the polarization states in
time slots (1) and (3) are the same, the phase is changed by .pi.,
i.e., inverted. Similarly, the polarization states in time slots
(2) and (4) are the same, but the phase is inverted. This
modulation method is different from that described above in this
regard.
The conditions for obtaining a perfect polarization switching light
source are described above. Three modulation methods which satisfy
the conditions will be described below with reference to FIGS. 6A
to 6D, FIGS. 7A to 7C, and FIGS. 8A to 8C, respectively. Assume
that the arrangement of an optical system employed in each case is
the same as that shown in FIG. 4. FIGS. 6A, 7A, and 8A respectively
show laser modulation current waveforms. FIGS. 6B, 7B, and 8B
respectively show phases .theta..sub.A (t) and .theta..sub.B (t) of
light electric fields synthesized by the fiber-optic polarization
beam splitter 48. FIGS. 6C, 7C, and 8C respectively show changes in
polarization state on a PLQR circular cross section of the Poincare
sphere. In each case, a phase difference probability density
function Pr(.DELTA..theta.) shown in FIG. 6D is obtained, and hence
R=0. Therefore, two orthogonal polarization states equally appear
in a transient state between 0 and .tau.. However, phase inversion
occurs between time slots (2) and (4). In the cases shown in FIGS.
6A to 6C, 7A to 7C, and 8A to 8C, .delta..phi.=0 is assumed.
However, similar to the above-described case, polarization
switching can be realized even if .delta..phi..noteq.0.
In the above-described cases, two independent polarization states
are switched. However, three or more polarization states can be
switched. FIGS. 9A and 9B show a case wherein three or more
polarization states are switched. In this case, phase modulation of
2/3.pi. is performed with one pulse. In addition, the propagation
delay between the two optical paths is not limited to .tau./2.
FIGS. 10A and 10B show a case wherein the propagation delay is set
to be .pi./4. FIGS. 9A and 10A respectively show laser modulation
current waveforms. FIGS. 9B and 10B respectively show phases
.theta..sub.A (t) and .theta..sub.B (t) of light electric fields
synthesized by the polarization beam splitter 48. In addition to
these modifications, various modifications of the modulation method
can be made.
Second Embodiment: Optical Receiver
An embodiment of an optical receiver according to the second aspect
of the present invention will be described below.
FIG. 11 shows the arrangement of a heterodyne optical receiver of a
100 Mb/s FSK single filter detection scheme. A polarization
switching light source 100 as a local oscillation source has an
arrangement substantially the same as that shown in FIG. 4. Output
light from the local oscillation source 100 is input to a 1:1
optical coupler 102 through an optical fiber 49. Reception signal
light is input to the 1:1 optical coupler 102 through an optical
fiber 101. The output light from the local oscillation source 100
and the signal light are mixed with each other at a ratio of 1 to 1
and are heterodyne-received by a balanced optical receiver 104
through optical fibers 103a and 103b. At this time, the optical
coupler 102 serves as a 180.degree. hybrid circuit. The balanced
optical receiver 104 comprises a light receiving portion
constituted by a series circuit of two photodiodes 105a and 105b,
and an intermediate frequency (IF) band front-end amplifier 106 for
receiving an output from a node between outputs from the
photodiodes 105a and 105b. The photodiodes 105a and 105b have
substantially the same characteristics in terms of, e.g., quantum
efficiency, response speed, and dark current, and are respectively
coupled to the optical fibers 103a and 103b with substantially the
same coupling efficiency .eta.=.eta.a=.eta.b. The following
components are connected to the output of the IF amplifier 106: a
bandpass filter 107, a mixer 108, a signal branch circuit 109, a
.tau./2-delay circuit 110, a low-pass filter 111, and an analog
adder 112. Part of the output of the bandpass filter 107 is coupled
to an IF automatic frequency control (AFC) circuit 113. The AFC
circuit 113 maintains an IF frequency constant by controlling a
current to a semiconductor laser 41 in the local oscillation source
100 through a laser driver 2.
An operation of the light receiver of this embodiment will be
described next. Polarization states are switched for every 5-ns
sub-time slot obtained by dividing a one-bit time slot T=10 ns into
two portions. Polarization switching is realized by the modulation
method shown in FIGS. 3A to 3L. For the sake of descriptive
convenience, assume that no polarization rotation occurs in the
optical fiber 49, and that a polarization coordinate system OXY of
the optical coupler 102 corresponds to that of the polarization
switching light source in FIGS. 3A to 3L. In this case, an x
component E.sub.Lx and a y component E.sub.Ly of an electric field
strength E.sub.L of the local oscillation light are respectively
given by ##EQU18## for ##EQU19## where N is an integer. In this
embodiment, .tau.=10 ns. Note that the switching time is neglected
here for the sake of descriptive convenience. The effects of the
switching time will be considered later.
An x component E.sub.Sx and a y component E.sub.Sy of the
photoelectric field strength ES of the signal light input from the
optical fiber 101 to the optical coupler 102 are respectively given
by
where 0.ltoreq..xi..ltoreq..pi./2 represents the polarization state
of the signal light. Assume that if a signal value S is "1",
.omega..sub.S (t) becomes .omega..sub.S1, and that if S="0",
.omega..sub.S (t) becomes .omega..sub.S2. In this case,
.delta..theta..sub.L and .delta..theta..sub.S are phase differences
between the x and y components of the local oscillation light and
the signal light, respectively. Assume that .omega..sub.L is
controlled by the AFC circuit 113 to have a predetermined frequency
relationship with .omega..sub.S1, i.e., .omega..sub.IF1
=.omega.L-.omega..sub.S1. In this case, a frequency deviation
.omega..sub.S1 -.omega..sub.S2 is set to be 1 GHz: and an
intermediate frequency .omega..sub.IF1, 1 GHz.
The photodiode 105 does not respond to a light electric field of
.omega..sub.S +.omega..sub.L. In addition, the x and y components
of an electric field do not interfere with each other. Since the
optical coupler 102 serves as a 180.degree. hybrid circuit, the
in-phase components of output currents from the photodiodes 105a
and 105b are removed (balanced optical receiver). The bandpass
filter 107 passes a component of .omega..sub.IF =.omega..sub.IF1 (1
GHz) but does not pass a component of .omega..sub.IF
=.omega..sub.IF2 (2 GHz). Therefore, when S=1, an output v.sub.If
from the bandpass filter 107 is represented as follows: ##EQU20##
where A is a constant, and .delta..theta.=.delta..theta..sub.L
-.delta..theta..sub.S. In this case, the sign "+" is given to the
solution if f(t)=0(5ns<t-NT<10ns); and the sign "-", if
f(t)=1(0ns<t-NT<5ns). The output v.sub.IF is then subjected
to square-law detection in the mixer 108 and is branched by the
signal branch circuit 109 on the basis of the value of f(t). When a
signal branching operation based on f(t)=1 is to be performed, the
circuit 109 is connected to the .pi./2 delay circuit 110. If each
branch component is filtered by the low-pass filter 111, the
following detection output can be obtained:
The cutoff frequencies of the low-pass filter is set to be 400 MHz,
an intermediate value between 200 MHz (1/T.sub.S) and 1 GHz
(.omega..sub.IF1 /2.pi.). The sign "+" represents a branching
operation based on f(t)=1; and the sign "-", a branching operation
based on f(t)=0. When both the outputs ar added by the analog adder
112, the resulting output is given by
thus obtaining an output independent of the polarization state of
the signal light. That is, polarization switching reception is
achieved. In this case, the power penalty caused by detecting only
a signal component having one polarization state in each sub-time
frame is 3 dB. When the signal value S=0, since the bandpass filter
output becomes 0, the output from the adder 112 becomes 0.
According to the method of this embodiment, signal reception
independent of polarization can be realized without using a
polarization diversity module which requires cumbersome operations
for adjustment and assembly and increases the cost. Although the
electronic circuit is slightly complicated, the apparatus of the
present invention can achieve a reduction in cost by forming it
into an IC. Therefore, the cost of the apparatus can be greatly
reduced as compared with the case wherein the polarization
diversity module is used. In addition, since adjustment and
assembly are facilitated, the apparatus of the present invention is
suitable for mass production.
The influences of the switching time, which is previously
neglected, will be considered now. As described with reference to
the first embodiment, pulse-like frequency modulation accompanying
polarization switching is caused in a beat output. Unless the
modulation method is modified, polarization states in one time slot
during a switching operation are not balanced. In this embodiment,
therefore, the signal branch circuit 109 is designed not to pass a
signal during a switching operation. Such an operation can be
realized by, e.g., adjusting the duty of a gate pulse output from a
clock control circuit 114. Although a penalty is caused in
accordance with a reduction in reception energy due to a signal, a
stable operation can be realized. In this embodiment, the time
required for switching, including pulse jitter, is about 250 ps.
The signal branch circuit 109 is designed to pass a signal only
during T.sub.S '=4.6 ns corresponding to a sub-time slot T.sub.S =5
ns from which two end portions are removed. The penalty based on
the fact that signal power corresponding to this portion cannot be
used is 0.4 dB or less. Note that .omega..sub.L is controlled by
the AFC circuit 113 to have a predetermined frequency relationship
with .omega..sub.S1, i.e., .omega.IF1=.omega..sub.L
-.omega..sub.S1. The AFC circuit 113 is controlled by a gate signal
output from the clock control circuit 114 to be operated only when
the output signal is "1". Similarly, in this case, the duty of a
gate pulse output from the clock control circuit 114 is adjusted to
prevent a frequency variation in polarization switching from
causing an unstable AFC operation.
In the above-described case, the time slot T is divided into two
portions. However, the time slot T may be divided into three or
more portions. If the time slot is divided into six or more
portions, polarization switching can be regarded as polarization
scrambling and the components from the signal branch circuit 109 to
the adder 112 can be replaced with one low-pass filter. More
specifically, by setting the time constant of the low-pass filter
to be a value between the length of the sub-time slot T.sub.S and
that of the time slot T, averaging, i.e., addition, of the
respective sub-time slots can be performed.
In this embodiment, FSK is performed. However, the present
invention can be applied to other modulation methods, such as DPSK
and ASK, and other detection methods, such as delay detection.
Third Embodiment: Optical Receiver
An embodiment of an image removing light receiver according to the
third and fourth aspects of the present invention will be described
below.
FIG. 12A shows the arrangement of a polarization diversity
heterodyne optical receiver of a 200 Mb/s DPSK differential
detection scheme of an embodiment according to the third aspect of
the present invention. This optical receiver is designed to
selectively receive signals, of light signals subjected to optical
FDM at intervals of 2.5 GHz, which are transmitted through a fiber.
The phase of output light from a semiconductor laser local
oscillation source 120 changes by .pi./2 for every sub-time slot
T.sub.S =1 ns, which is obtained by dividing a time slot T=5 ns
into five equal portions. However, no phase switching occurs in the
boundary between adjacent time slots. This operation can be
realized by performing pulse-like direct frequency modulation in
the boundary between adjacent sub-time slots by using a laser
driver 121 according to equation (2). The time required for phase
switching is 0.1 ns. Local oscillation light is incident on a
polarization beam splitter 122a at an angle of 45.degree. with
respect to its optical axis to be split into two components having
the same power. Similarly, signal light is split into two polarized
light components by a polarization beam splitter 122b. The signal
light components and the local oscillation light components are
respectively mixed by fiber couplers 123a and 123b while their
polarization states are matched with each other. The mixed light
components are respectively received by balanced optical receivers
124a and 124b. The arrangement of each light receiver is
substantially the same as that of the optical receiver of the
second embodiment. An output from each of bandpass filters 125a and
125b connected to the outputs of the respective optical receiving
units is divided into two branch outputs. One branch output is
delayed with respect to the other branch output by T/5, while the
phase of the other branch output is delayed by 90.degree. by a
90.degree. phase shifter 126a or 126b. Subsequently, one branch
output and the other branch output are added by an adder 127a or
127b. An output from each of the adders 127a and 127b is divided
into two portions. After one branch output is delayed by T, the two
outputs are multiplied with each other by a mixer 128a or 128b. The
resulting value is then introduced into a low-pass filter 129a or
129b. Finally, the two polarized light components are added by an
adder 130 to obtain a demodulated signal. In addition, part of the
output of each bandpass filter 125a or 125b is coupled to an IF
automatic frequency control (AFC) circuit 113. The AFC circuit 113
controls the oscillation frequency of the local oscillation light
source 120 through the laser driver 121 so as to keep an IF
frequency constant.
An operation of the light receiver of this embodiment will be
described next. Signal light includes a plurality of channels. In
this case, only a component having an angular frequency
.omega..sub.S1 to be received, and a component having an angular
frequency .omega..sub.S2 in a channel adjacent to the opposite side
of local oscillation light having an angular frequency
.omega..sub.L will be considered. Assume that the light frequencies
are set to be
Other components need not be considered because beating occurs
outside the band of each light receiving unit. An output from the
local oscillation source 120 is phase-modulated by .pi./2 four
times in the time slot T by the laser driver 121. The time slot is
divided into five sub-time slots, each having a length T.sub.S =1
ns. The polarization beam splitter 122a or 122b, the fiber coupler
123a or 123b, and the balanced optical receiving unit 124a or 124b
receive beat signals produced by the respective polarized light
components of the local oscillation light and the signal light in
each sub-time slot. The phase of the received beat signal is
shifted by .pi./2 for every sub-time slot. According to the
expression used in the above description, beat outputs produced by
branch components of polarized light a in the respective sub-time
slots are given as follows: ##EQU21## where S.sub.i is the phase
value of a time slot i, which is set to be 1 or -1 (a state wherein
the phase is inverted by .pi.), A.sub.1, A.sub.2, .theta..sub.1a,
and .theta..sub.2a are constants, and .xi..sub.1a and .xi..sub.2a
(0.ltoreq..xi..sub.1a, .xi..sub.2a .ltoreq.1) are the ratios of the
polarized light a in signal light electric fields in channels 1 and
2. Branch components of polarized light b correspond to outputs
obtained by replacing a with b in the above equations. However,
.xi..sub.1a.sup.2 +.xi..sub.1b.sup.2 =1 and .xi..sub.2a.sup.2
+.xi..sub.2b.sup.2 =1.
This beat output is divided into two branch beat outputs. One
branch beat output is then delayed with respect to the other branch
beat output by T/5, while the phase of the other branch beat output
is shifted by .pi./2 by the phase shifter 126a. The two outputs are
added by the adder 127a. Adder outputs X1 are given by ##EQU22##
Thus, image components are removed. Since X.sub.5a is obtained by a
calculation with the next time slot component, it is not used for
signal detection. The corresponding power penalty is about 1 dB.
Since the IF frequency is 1 GHz, one period of the IF signal
corresponds to the length T.sub.S (=1 ns) of the sub-time slot. In
this case, if the output in each sub-time slot is differentially
detected by the delay line of T, the mixer 128a, and the low-pass
filter 129a, the resulting output is represented by ##EQU23##
Similarly, with regard to the branch component, of the polarized
light b, which has a polarization state orthogonal to that of the
branch component of the polarized light a, the following output can
be obtained:
Therefore, if the two outputs are added by the adder 130, since
.xi..sub.1a.sup.2 +.xi..sub.1b.sup.2 =1,
is obtained. If it is determined whether S.sub.i S.sub.(i+1) is 1
or -1, a signal value can be identified. That is, polarization
diversity image rejection signal reception is achieved.
According to the optical receiver of the third embodiment, the
optical system can be greatly simplified as compared with the
polarization diversity image rejection light receiver shown in FIG.
18. In comparison with the optical system which is high in cost and
requires cumbersome operations for adjustment and assembly, an
electronic circuit allows a great reduction in labor and cost.
Therefore, a considerable reduction in cost can be achieved as a
whole. In addition, the performance of the apparatus can be
improved in terms of temperature characteristics and stability of
operation. Signals corresponding to portions of the switching time
and AFC can be treated in the same manner as in the second
embodiment.
FIG. 12B shows a case wherein phase switching of the local
oscillation source is alternately performed between .pi./2 and
-.pi./2. Although the basic arrangement of the local oscillation
source is similar to that shown in FIG. 12A, the portions enclosed
within an alternate long and short dashed line in FIG. 12A is
replaced with the portion shown FIG. 12B. Four sub-time slots are
used, TS=1.25 ns, and the IF frequency is 800 MHz. Signal switching
circuits 131a and 131b are respectively arranged in front of the
phase shifters 126a and 126b. Under the same conditions as in the
above-described embodiment, the following outputs appear at the
output of the optical receiving unit in each sub-time slot of the
branch component of the polarised light a: ##EQU24## This beat
output is divided into two branch components. One branch component
is delayed with respect to the other branch component by T/4. The
branch component whose phase is shifted by .pi./2 by the phase
shifter 126a is switched for every sub-time slot by the signal
switching circuit 131a. When the two branch outputs are added by
the adder 127a, the output X.sub.1 is given by ##EQU25## Thus,
image components are removed. Since X.sub.4ai is obtained by a
calculation with the next time slot component, it is not used for
signal detection. Since the IF frequency is 800 MHz, one period of
the IF signal corresponds to the length T.sub.S (=1.25 ns) of the
sub-time slot. Subsequent operations are the same as those in the
embodiment shown in FIG. 12A, and
is obtained as an output from the adder 130. In addition, the same
effect as in the light receiver shown in FIG. 12A is obtained.
Fourth Embodiment: Coherent Optical Transmission System
An embodiment of a coherent optical transmission system for
simultaneously performing phase switching and polarization
switching according to the fifth aspect of the present invention
will be described in detail below with reference to FIGS. 13A and
13B.
FIG. 13A shows the schematic arrangement of an optical FDM
high-definition CATV distribution system according to this
embodiment. In a wavelength band of 1.55 .mu.m, 200-Mb/s DPSK
signals are transmitted in 64 channels at channel intervals of 6
GHz. The wavelength band used in this system is about 30 .ANG..
Each light transmitter 140 includes a semiconductor laser module
141, a driver 142, and a stabilizing circuit 143. The driver 142
includes a bias circuit 144, a DPSK coder 145, a phase switching
circuit 146, and the like. Light signals of the respective channels
are mixed with each other by a star coupler 147 and are distributed
to tens of thousands of subscriber's optical receivers 150 through
1:8 light distributors 148 and optical amplifiers 149 for
compensating for losses accompanying the distribution.
Synchronization of modulation timings, the channel interval, and
the absolute frequency are controlled by a common control system
151.
FIG. 13B shows the arrangement of each subscriber's optical
receiver. The arrangements of a polarization switching local
oscillation source 152, a photo-coupler 153, and a balanced optical
receiving unit 154 are the same as those in the optical receiver of
the second embodiment. An output from the balanced optical
receiving unit 154 is differentially detected by a differential
detector 155. The detection output is converted into a digital
value by an A/D converter 156 and is input to four sample/hold
circuits 157 having different sampling timings. Outputs from these
circuits 157 are then added by a sample/hold circuit 158 arranged
after the circuits 157. The addition result is output to an
identifying circuit 159. In addition to these components, the
optical receiver 150 includes a clock circuit 160 for controlling
the timings of the sample/hold circuits and an AFC circuit 161 for
stabilizing the oscillation frequency of a local oscillation
source.
An operation of this coherent optical transmission system will be
described next.
The optical transmitter 140 performs 0- or .pi.-phase pulse
modulation of a semiconductor laser through the DPSK coder 145. In
addition, .pi./2-phase fixed phase modulation is also performed
every 2.5 ns corresponding to 1/2 a time slot T=5 ns by means of
the phase switching circuit 146. With this operation, the time slot
is divided into two phase time slots T.sub.1 and T.sub.2. This
phase modulation is performed by decreasing the optical frequency
in the form of a pulse. The timings of this modulation are
controlled by a common control system 151 so as to coincide with
each other in all the channels when light signals are mixed by the
star coupler 147. Therefore, in the subscriber's optical receivers
150, frequency variations accompanying phase modulation
simultaneously occur in the respective channels. If a signal value
in a given time slot j of a channel i is represented by A.sub.ij (1
or -1), the electric field strengths of light outputs in the time
slots T.sub.1 and T.sub.2 are respectively represented by A.sub.ij
.vertline.E.sub.Si /.sqroot.2.vertline.cos(.omega..sub.Si
t+.theta..sub.Si) and A.sub.ij .vertline.E.sub.Si
/.sqroot.2.vertline.sin(.omega..sub.Si t+.theta..sub.Si). Since the
DPSK modulation signal is subjected to .pi./2 phase switching
twice, if no modulation signal is supplied from the DPSK coder 145,
A.sub.ij and A.sub.i(j+1) are reversed, and vice versa.
Output light from the local oscillation source 152 of the light
receiver 150 alternately exhibits orthogonal polarization states
upon polarization switching. These polarization states respectively
define polarization sub-time slots T.sub.A and T.sub.B. The
polarization sub-time slots are set to be switched at an
intermediate position in a phase sub-time slot. In addition, phase
modulation for polarization switching is realized by decreasing the
optical frequency of the local oscillation source 152 in the form
of a pulse by using the modulation method shown in FIGS. 5a to 5C.
Therefore, when the output light exhibits the same polarization
state again, the phase is inverted. In a polarization sub-time slot
A, output light exhibiting a polarization state A with an electric
field strength .vertline.E.sub.L
/.sqroot.2.vertline.cos(.omega..sub.L t+.theta..sub.LA) appears. In
a polarization sub-time slot B, output light exhibiting a
polarization state B with an electric field strength
.vertline.E.sub.L /.sqroot.2.vertline.cos(.omega..sub.L
t+.theta..sub.LB) appears. Assume, in this case, that the local
oscillation source is set to satisfy .omega..sub.L =.omega..sub.Si
in order to select the channel i from the channels of signal
light.
This local oscillation light is mixed with the signal light by the
optical coupler 153, and the resulting light is received by the
balanced optical receiver 154. If the components, of the signal
light power in the channel i, which have the polarization states A
and B are respectively represented by .xi..sub.iA.sup.2 and
.xi..sub.iA.sup.2, .xi..sub.iA.sup.2 +.xi..sub.iB.sup.2 =1,
0.ltoreq..xi..sub.iA .ltoreq.1, and 0.ltoreq..xi..sub.iB .ltoreq.1.
At this time, in polarization/phase sub-time slots corresponding to
phase sub-time slots 1 and 2 and the polarization sub-time slots A
and B, the following beat output components appear: ##EQU26## where
P is a constant representing a loss and a conversion efficiency,
and .delta..theta..sub.iA and .delta..theta..sub.iB are the phase
differences between signal light and local oscillation light of the
respective polarization components. When these outputs are
multiplied by a component in a time slot (j+1) and differentially
detected by the differential detector 15, the following values are
obtained: ##EQU27##
The sign "-" is produced when the phase is inverted by 180.times.
when the polarization switching shown in FIGS. 5A to 5C is
performed. These outputs are converted into positive/negative
octonary values by the A/D converter 156. The input level of the
A/D converter 156 can be kept constant by controlling the gain of
the amplifier of the balanced light receiving unit 154. Since these
outputs appear time-serially in the order named, they are held by
the sample/hold circuits 157 having different sampling timings
until the last output X.sub.ij2A appears. When the last output
X.sub.ij2A appears, all the outputs are input to the sample/hold
circuit 158 to be added. As a result, the following value is
obtained by the identifying circuit 159: ##EQU28## Therefore,
signal reception can be performed independently of the phase
difference between signal light and local oscillation light and the
polarization state of signal light in the optical receiver. The
circuits subsequent to the A/D converter 156 are operated by a
clock of 800 MHz. This operation can be realized by an Si bipolar
IC. Although a quantization error appears with respect to each
sub-time slot in the A/D converter 156, since it is only required
in the last identifying step to determine whether A.sub.ij
A.sub.i(j+1) is 1 or -1, a quantization level of about an octonary
value is enough. In practice, the polarization state of a light
source varies depending on the state of an optical fiber. However,
such variation is slow as compared with the time slot T and hence
poses no problems. Although the phase also varies with time due to
phase noise, a large penalty can be prevented as long as phase
variation in the time slot T is sufficiently small. In addition, if
the intermediate frequency is sufficiently small as compared with
the frequency (800 MHz) of the sub-time slot, it need not be
completely zero. In the above-described case, A/D conversion is
performed before the respective time slot outputs are added.
However, these outputs may be added as analog signals.
The clock circuit 160 performs clock extraction on the basis of the
frequency variation of an output signal from the balanced optical
receiver 154. That is, since a high-frequency beat component
accompanying a frequency variation appears upon switching of
sub-time slots, clock extraction can be performed on the basis of
the timing that the beat component appears. Since a frequency
variation changes depending on the value of a DPSK signal upon
switching of time slots, the frequency variations of local
oscillation light and signal light can be identified. Furthermore,
by detecting the interval between the frequency variation of signal
light and that of local oscillation light, the timing of
polarization switching can be controlled.
Similar to the second embodiment, an error accompanying a switching
operation can be prevented by performing timing control to inhibit
an operation of the A/D converter 156 at the instant when a beat
frequency variation accompanying the phase variation of signal
light or local oscillation light appears. Similarly, the frequency
variation of the local oscillation source can be suppressed by
inhibiting the use of a beat signal in a switching state for AFC of
the local oscillation source. Since the timings of phase switching
are the same in all the channels, no error is caused in a given
channel due to the influences of a frequency variation in another
channel.
According to the coherent transmission system of this embodiment,
with one optical receiving unit, the same function as that of a
polarization/phase-diversity light receiver using four optical
receiving units can be achieved. Although the operation speed of
the optical receiver is increased, since a high intermediate
frequency need not be used because of phase switching, no
significant problems are posed. The system requires a complicated
electronic circuit. However, an increase in cost can be suppressed
by the use of ICs. Therefore, the complicated optical system can be
simplified, and the number of optical receiving units can be
reduced. Such merits are much worthier than the above-mentioned
demerits. In multi-port reception, the characteristics of the
respective optical receiving units must be matched with each other.
In this embodiment, however, since one optical receiving unit is
used time-divisionally, no such consideration need be given.
Therefore, a subscriber's terminal with a small number of portions
to be adjusted can be realized at a low cost, and a large CATV
distribution system can be formed.
The present invention is not limited to this embodiment but can be
applied to, e.g., an optical subscriber's system, a trunk
transmission system, an optical LAN, a MAN, and an optical
switching system.
Fifth Embodiment: Coherent Optical Transmission System
An embodiment of a coherent optical transmission system for
simultaneously performing image rejection and polarization
switching operations according to the sixth and seventh aspects of
the present invention will be described in detail below with
reference to FIG. 14.
An optical FDM high-definition CATV distribution system according
to an embodiment of the present invention has substantially the
same arrangement as that of the system shown in FIG. 13A. In this
system, 150-Mb/s DPSK signals are transmitted in 64 channels at
channel intervals of 4.5 GHz in a wavelength band of 1.55 .mu.m.
The arrangement of each light transmitter 140 is the same as that
in the fourth embodiment except that five phase sub-time slots are
present, and the phase changes by .pi./2. Although frequency
modulation for DPSK is performed upon switching of time slots, no
phase switching is performed.
FIG. 14 shows the arrangement of a subscriber's optical receiver
201. The arrangements of a polarization switching local oscillation
source 202, an optical coupler 203, a balanced optical receiving
unit 204, and the like are the same as those in the light receiver
of the fourth embodiment. Polarization switching is performed at a
frequency twice that used in the fourth embodiment in synchronism
with each phase sub-time slot of signal light by the method
described with reference to FIGS. 3A to 3F. Therefore, a total of
10 phase/polarization sub-time slots are formed. Although signal
light includes a plurality of channels, only a component having an
angular frequency .omega..sub.S1 to be received, and a component
having an angular frequency .omega..sub.S2 in a channel adjacent to
the opposite side of local oscillation light having an angular
frequency .omega..sub.L will be considered hereinafter. Assume that
the light frequencies are set to be (.omega..sub.S1
-.omega..sub.L)/(2.pi.)=1.5 GHz and .omega..sub.L -.omega..sub.S2
/(2.pi.)=3 GHz. Other components need not be considered because
beating occurs outside the band of the optical receiving unit. The
IF frequency of a signal is synchronized with the switching
frequency (1.5 GHz) for the sub-time slots. Beat signals resulting
from the respective polarization components of local oscillation
light and signal light are received in units of sub-time slots. The
beat outputs in the respective sub-time slots are ##EQU29## An
output from a bandpass filter 205 arranged after the light
receiving unit 204 is divided into two branch components. One
branch component is delayed with respect to the other branch
component by T/5, while the phase of the other branch component is
delayed by 90.degree. by a phase shifter 206. The two branch
outputs are then added by an adder 207. As a result, similar to the
third embodiment, the following values are obtained: ##EQU30##
Thus, image components are removed. Since X.sub.5a and X.sub.5b as
sub-time slot outputs are obtained by calculations with the next
time slot component, they are not used for signal detection. The
corresponding power penalty is about 1 dB. Since the IF frequency
is 1.5 GHz, one period of the IF signal corresponds to the length
T.sub.S of the sub-time slot. In this case, if the output in each
sub-time slot is differentially detected by a delay line of T, a
mixer 208, and a low-pass filter 209, the resulting outputs are
represented by ##EQU31## Since orthogonal polarization states a and
b alternately appear, if the two outputs are added by an adder 210,
since .xi..sub.1a.sup.2 +.xi..sub.1b.sup.2 =1, the following value
is obtained:
If an identifying unit 211 is used to determine whether S.sub.i
S.sub.(i+1) is 1 or -1, a signal value can be identified. That is,
polarization switching/image rejection reception can be performed.
Note that the same function as that of addition can be realized by
averaging without using the adder 210 by increasing the time
constant of the low-pass filter 209.
Part of the output of the bandpass filter 205 is coupled to an IF
automatic frequency control (AFC) circuit 213. The AFC circuit 213
controls the oscillation frequency of a local oscillation light
source 202 through a laser driver so as to keep the IF frequency
constant.
According to this embodiment, image removing signal reception can
be realized independently of polarization with a simple optical
system, thus realizing an optical FDM with a reduced channel
interval.
Phase switching may be alternately performed between .pi./2 and
-.pi./2. Although the basic arrangement is similar to that shown in
FIG. 13A, this embodiment employs four phase sub-time slots and
eight polarization time slots and eight polarization/phase sub-time
slots. In addition, the IF frequency is set to be (.omega..sub.S1
-.omega..sub.L)/2=1.2 GHz and (.omega..sub.L
-.omega..sub.S2)/(2.pi.)=2.4 GHz, and the frequency interval is 3.6
GHz. A signal switching circuit 214 is arranged before the phase
shifter 206. Under the same conditions as those in the
above-described embodiment, the following outputs appear at the
output of the light receiver in the respective sub-time slots:
##EQU32## This beat output is divided into two branch components.
One branch component is delayed with respect to the other branch
component by T/4. The branch component whose phase is shifted by
.pi./2 by the phase shifter 206 is switched for every sub-time slot
by the signal switching circuit 214. When the two branch outputs
are added by the adder 207, outputs X are given by ##EQU33## Thus,
image components are removed. Since X.sub.4ai and X.sub.4bi are
obtained by calculations with the next time slot component, they
are not used for signal detection. Since the IF frequency is 1.2
GHz, one period of the IF signal corresponds to the length T.sub.S
of the sub-time slot. Subsequent operations are the same as those
in the embodiment described above, and
is obtained as an output from the adder 210.
In addition to this embodiment, the phase switching method, the
modulation method for polarization switching, and the combination
thereof can be variously modified and applied.
As has been described in detail above, according to the present
invention, since the polarization switching light source according
to the first aspect of the present invention employs direction
modulation of a semiconductor laser as a light source, the
insertion loss and the number of portions to be adjusted can be
reduced as compared with the case wherein an external modulator is
used. Therefore, mass production, a cost reduction, and reductions
in size and weight of the system can be achieved. In general, an
external modulator, either a phase modulator or a polarization
modulator, based on the electrooptic effect requires a high driving
voltage, leading to difficulty in high-speed switching. In contrast
to this, since a semiconductor laser easily allows a high-speed
operation and has a frequency modulation efficiency on the order of
1 GHz/mA, phase modulation of .pi./2 or .pi. can be performed with
a current pulse of several mA and about 100 ps. Therefore,
high-speed switching can be easily achieved. In addition, the
reliability of switching is high because an external modulator
having poor temperature characteristics and low reliability is not
used, and a semiconductor laser for a light source is generally
stabilized against temperature changes and has high
reliability.
In polarization switching based on the modulation method of the
present invention, the oscillation frequency and output of the
laser are kept constant except for a region corresponding to a
short period of time during which a pulse current is supplied.
Therefore, changes in the oscillation frequency and output of the
laser before and after switching, which pose the problems in the
conventional polarization switching, do not occur. Therefore, the
application range of the system is wide.
The arrangement of the optical receiver according to the second
aspect of the present invention is simpler than that of the
conventional polarization diversity optical receiver, thereby
allowing a reduction in the number of portions to be assembled and
adjusted, and realizing mass production and reductions in cost,
size, and weight. In addition, the reliability can be improved. In
the conventional multi-port reception, the characteristics of the
respective ports must be matched with each other. In the method of
the present invention, however, since one receiver is used
time-divisionally, no characteristic variation basically occur in
reception of different polarization states. Furthermore, in the
conventional polarization switching (polarization scrambling),
since no external modulator is used, the light receiver of the
present invention realizes high-speed switching, a low loss, high
stability, a low cost, and reductions in size and weight.
Therefore, the optical receive is advantageous in terms of
reliability and mass production. Moreover, since no frequency
variation of local oscillation light occur before and after
polarization switching, the AFC circuit is free from complication.
In performing FSK, the IF band and the modulation factor need not
be increased exceedingly.
The arrangement of the optical receiver according to the third and
fourth aspects of the present invention is simpler than that of the
conventional image rejection optical receiver. Therefore, the
number of portions to be assembled and adjusted can be reduced, and
mass production and reductions in cost, size, and weight can be
realized in addition to a improvement in reliability. Furthermore,
in the conventional multi-port reception, the characteristics of
the respective ports must be matched with each other. In the method
of the present invention, however, since one receiver is used
time-divisionally, no characteristic variation basically occur in
reception of different branch light components.
The coherent optical transmission system according to the fifth
aspect of the present invention has a simple arrangement as
compared with the coherent optical transmission system using the
conventional polarization/phase diversity optical receiver. The
coherent optical transmission systems according to the sixth and
seventh aspects of the present invention have simple arrangements
as compared with the conventional coherent optical transmission
system using the image rejection optical receiver. In either of the
aspects described above, the number of portions to be assembled and
adjusted can be reduced, and mass production and reductions in
cost, size, and weight can be realized. In addition, in the
conventional multi-port reception, the characteristics of the
respective ports must be matched with each other. In the method of
the present invention, however, since one receiver is used
time-divisionally, no characteristic variation basically occur in
reception of different polarization states and different phase
states.
According to the present invention, therefore, (1) a polarization
switching light source having a simple arrangement and a small
number of limitations can be realized, (2) image rejection signal
reception based on new concepts, which prevents complication of an
optical system, can be realized, and (3) coherent optical reception
with a simple optical system, which is independent from
polarization and resistant to phase noise, can be achieved.
Therefore, the present invention can solve the problems of
polarization matching, phase noise, receiver band width, and
high-density optical FDM, which interfere with the practical
applications and spread of coherent optical communication, at low
cost.
Additional advantages and modifications will readily occur to those
skilled in the art. Therefore, the invention in its broader aspects
is not limited to the specific details, and representative devices,
shown and described herein. Accordingly, various modifications may
be made without departing from the spirit or scope of the general
inventive concept as defined by the appended claims and their
equivalents.
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